A 2010 iMac has only USB 2.0, so trying to copy anything to an external drive via USB will take an extremely long time.
Longer than USB 3 which isn't an option here, yes but extremely long? Nonsense.
I can only assume that you've not done much of that lately—but why would you unless you don't have a 2012 or later Mac? Yes, it takes awhile but Firewire isn't much faster — really, I have old FW 800/USB 2 housings still and have timed it.
Enough of opinions. Let's get to facts. I was planning to test USB 3 vs USB-C on my iMac Pro today but my 2010 is still on the desk so it was very easy to add USB 2.
I used the same 25.57GB test file and two drives: a 2TB Crucial MX300 SSD and a 10 year old WD Black 500GB 7200 rpm system pull from an early 2009 iMac 24". Out of the hundreds I've replaced, not one OE HDD from a late 2009–2012 has ever tested good. Even if they still worked, all had serious SMART errors.
I used 3 docks, the USB 3 I referred to earlier, an 9 year old eSATA/USB 2 and a new USB3/USB-C I got last week — I needed another anyway and was curious to see if USB-C made a difference.
https://www.amazon.com/gp/product/B01E80N2E8/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
I ran 12 tests but I'll spare you the blow-by-blow. The conclusions were few and explanations simple.
USB 2: transfer time 9 minutes =/- 4 seconds. The drive didn't matter nor the dock if I was using a USB 2 cable. The limitation was clearly USB 2.
USB 3: HDD: 3 minutes 56 seconds; SSD: 1 minute 10 seconds +/- 3 seconds. Now we hit the platter speed of the HDD and that explains the difference.
USB-C: Identical to USB 3. The reason is that SATA III is the limiting factor, slower than either USB 3 or USB-C.
eSTA (2010 iMac only) HDD: 3 minutes 56 seconds. SSD: 1 minutes 55 seconds. The platter speed is still the limit on the HDD but SATA II is now the limit for the SSD. If my 2010 had an HDD instead, the transfer time would likely be around 4 minutes.
BTW, this does not mean that an SSD is only twice the speed of an HDD inside a 2010, not at all. Bus speed does not affect the internal workings of the SSD. Boot times are seconds instead of minutes. Track bouncing in my DAW can take less than a second on small files instead of minutes on the HDD. 3 years with an HDD and 6 years with an SSD in the same iMac... Thanks to eSATA, I was able to compare both (a 1T 845 EVO) in the same machine simply by deciding which one to boot from. It wasn't till 2015 when Samsung released the 2TB 850 EVO that I finally installed it internally and dramatically lowered the temperature in my office. The reduction in my A/C bill during the summer paid for it over time. A week later, I installed the 845 into my wife's 2011 iMac.
You can still do eSATA on the 2010–2011 27" iMacs only. The drop-in docks are really inexpensive but the cost for conversion is $159 incl. return shipping from OWC plus shipping to them. When I had that done to mine in 2010, it made a lot of sense. Nowadays? Oh gosh no.
https://eshop.macsales.com/shop/turnkey/iMac_2010_27/add_eSATA
By comparison, one of the files that made up the test file is a 7.15GB restore DVD image file for the 2010–2011 iMac. I sent that file to someone's GoogleDrive account a few months ago. The upload took over 7 hours for that one file.