The USBC/TB ports on my new M1 Mini (8/500) are ridiculously slow at 40MB per sec reading and writing an external SATA SSD. Same SSD drive connected minutes before to the regular USB ports give 380 or so. I've tried multiple external SSD drives including the T5, and all give the same result. Installed Big Sur clean in a new partition, absolutely no difference. What's going on?
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M1 Mac mini Thunderbolt/3.1 speeds really really slow: 40MB per second!
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Incompatible controllers on the M1 and your drives. You might need a hub in between .
USB on M1 Macs isn't actually 10Gb/s? (Also definitely not USB4)
After quite a bit of testing, I have concluded that the USB bus on at least my Mac mini M1 does not actually support "USB 3.1 Gen 2 (up to 10Gb/s)" as listed on the Tech Specs page. I have a Samsung T5 USB-C SSD. It supports "USB 3.1 Gen 2 10Gb/s" (Gawd, I hate USB 3.1+ naming conventions...†)...
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I booted (to the password prompt) my M1 Mac Mini (16/512) from the internal SSD in the exact same amount of time as my external WD My Passport 500GB SSD, using the USB-C cable. That would imply that the internal SSD operates at no faster than the USB-C port, which obviously is not the case. Don't be down-trodden by benchmark numerics. If the system feels faster and does things faster, be happy. I sure know I'm not complaining! 
Looks as if you might be using a USB-C charging cable which are often only USB 2 speed for data.
OP: Those speeds are indicative of a USB 2.0 cable. My Crucial MX500 2TB SATA drive in a no-name USB 3.0 enclosure gets 223 MB/s write / 240 MB/s read on my M1 Mac mini.
The cable I've used is a standard short USB-C cable off Amazon connected from the Thunderbolt port to a Samsung T5. I later did try an Apple USB-C charge cable, which gave the same grim 40Mb rate as the standard USB-C cable. What rates are you folk getting with the same setup? What data rates have folk got using a Thunderboltexternal drive?
Get a proper USB 3 cable. There is no such thing as a standard USB-C cable! "USB-C" is just the name for the physical shape of the connector not what it does. It could be USB2, USB 3, Thunderbolt etc. All cables sold as charging cables including Apple's are USB 2.
Your Samsung T5 should have been supplied with a 10Gbps rated cable. Otherwise buy one from Amazon but make sure it says "10Gbps data transfer". It doesn't matter if it also says "charging cable" as long as it says 10Gbps.
I just checked Samsung T5 connected to my M1 MBA and it is 389/377 MB/s (R/W) with a 10Gbps cable.
BTW the Samsung T5 is not a Thunderbolt external drive, it is USB.
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A charge cable doesn't need to transmit data, so it may be a USB 2.0 cable.
Samsung T5 is not a Thunderbolt external drive. It's USB 3.1 gen 2. Some USB devices are not connecting to the USB controller of the M1 Macs at 10 Gbps speed. Therefore the solution is to connect the T5 to a different USB port. A USB 3.1 gen 2 hub may allow the T5 to work at 10 Gbps. However, in that case you are still using the USB controller of the M1 Mac which is reported to be slower than most other USB 3.1 gen 2 controllers (except the ASMedia ASM1142 which is limited to 8 Gbps). Therefore for max speed, connect the T5 to a different USB controller. Thunderbolt hubs and docks have their own USB controllers which are faster than the USB controllers of the M1 Macs.
Using the USB-C to USB-C cable that came with the Samsung T5, I get 493MB/s write and 526MB/s read on my M1 Mac mini.
Both cables that come with the Samsung T5 or T7 work as expected on my M1 Mac mini as well, attached via an OWC TB3 Dock or on my late 2013 “trashacan” Mac Pro via the CalDigit TS3+ Dock.
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I have the larger 2021 11-port OWC TB4 Dock. It's super slow with my M1 MBP. I have two Sansdisk SSDs (Extreme Pro USB 3.1 and 3.2) which both get 900+ MB/s (read/write) when directly attached the M1 MBP over USB-C. But through the dock I'm getting 150 MB/s. I verified this with ATTO Disk Benchmark too. It's also not a cable problem. I'm only using cables that came with the devices and I swapped them with another half-dozen from other USB-C devices; many are branded with the 10Gbps logo too.
When I attach that same TB4 Dock to my 2013 15" MBP over TB1 (TB2 > TB3 adapter) I get the Max for USB-A just under 600 MB/s. Seems the dock they built for the M1 has a compatibility problem with the M1.
Both drives perform normally when I use my CalDigit TS3 dock and my Akito TB3 Dock/RAID. Same if I put my 2013 15" in Target Disk mode, so that rules out the portable drives as the source of the problem.
Interesting, there must be more users with the same problem, I wonder if it’s a firmware problem from OWC or something that needs to be fixed Apples side?
It's definitely slower, but not that slow. I have a TB3 NVMe enclosure which I could get 2300 MB/s read and write on my desktop Mac, even through a TB hub with multitude of other devices connected to the same hub. But on my M1 MBA, I only get 1200MB/s write speed, though read remains 2300 MB/s.
Just discovered this thread whilst looking for something else so thought I would add my voice (even though it's not that related to the OP). I have just tried the smaller OWC Thunderbolt 4 Hub (3 TB ports + 1 x USB-A) and had issues with speed of external SSD's. I have a Crucial X8 and an NVMe in a USB-C enclosure - both get around 900/900 on my Belkin Thunderbolt 3 dock or connected direct to my Mac mini M1. But, on the OWC Hub, the speeds on both drop to around 600/600, which was disappointing. So it's going back.
Also, I don't know if this is normal, but I also have a fast NVMe in a Thunderbolt 3 enclosure, which when connected direct to the Mini gets about 2400/2400. But when connected to the Thunderbolt port on either the OWC or the Belkin, the write drops to about 1600 whilst the Read stays the same - is that normal on a Thunderbolt dock?
I encountered a similar issue on my intel MacBook Pro 16", where a thunderbolt 3 external SSD slows down to 40Mb/s in some scenarios depending on the order / side of the USB-C ports that I plug in. After some experiment, I have concluded that the slowdown happens when I first plug in one of the four ports with a USB-C to DP cable to a monitor, then plug in the external SSD into the other port on the same side. This leads to my suspicion that by first plug in some cable/device that uses usb2.0, the USB bus (at least the one on that side) is set to usb 2.0 mode, leading to later thunderbolt devices plugged into the same bus only using the usb 2.0 speed. Again, it is only a reasonable suspicion, and might explain the slowdown happened to your M1 Mac Minis.
USB 3 and USB 4 have thew same Speed! You kids don't seem to understand that! USB 4 was to allow Docks to make more strides in networking more ThunderBolt 4 ports!
USB4 is the same speed as USB3 when USB4 is doing USB3 stuff. USB4 can do faster stuff like Thunderbolt 3 and DisplayPort.
I'm not sure what point you're trying to make. That video compares Thunderbolt 3, Thunderbolt 4, and USB4. USB 3 is something else.
Why do you say USB3 and USB4 are the same speed when they do totally different things?
USB3 is 10 Gbps.
USB4, Thunderbolt 3, and Thunderbolt 4 are 40 Gbps but they all do USB at up to 10 Gbps. They can do PCIe at up to ≈23 Gbps. They can do DisplayPort at up to ≈34 Gbps. They can do combinations of all three up to 40 Gbps total.
I'm trying to educate the Thunderbolt 4 is everything Thunderbolt 3 should have been! Sure they're the same speed and all but 4 has to have it's own chip so every port has 40G bandwidth to 3 one thunderbolt controller for two ports!
You're talking about an implementation detail of Thunderbolt 4 that is not always true. For example, PC's that use Intel Maple Ridge Thunderbolt 4 host controller will have two Thunderbolt 4 ports per Thunderbolt bus that have similar shared bandwidth as a Thunderbolt 3 host controller. The two Thunderbolt 4 ports will be limited to PCIe 3.0 x4 speed (<31.5 Gbps) of PCIe data (but more likely ≈24 Gbps which is what I've measured for Titan Ridge Thunderbolt 3 host controller using two NVMe Thunderbolt enclosures and software RAID - one per port).
The Thunderbolt 3 buses of the Ice Lake based MacBook Pros are integrated in the CPU so they don't have the 31.5 Gbps PCIe limit (the limit is more like 41 Gbps for any combination of up to 4 ports). It still has two ports per bus though so you can only connect two displays - one per port of a bus, or two for one port of a bus. PCs with Tiger Lake are similar except they support Thunderbolt 4.
The Thunderbolt 4 buses of M1 Macs are also integrated in the CPU. Apple made each port its own bus so each port can connect up to two displays.
Anyway, the benefits you see with the Thunderbolt 4 implementation on M1 Macs is not because they are Thunderbolt 4. It's because Apple made them that way.
I suppose if you want equal capability with the discrete Thunderbolt 3 and Thunderbolt 4 controllers (Titan Ridge, Maple. Ridge), you could just expose one port per Thunderbolt controller, and connect multiple Thunderbolt controllers. It would use up 4 PCIe lanes per Thunderbolt controller though.