iMacs
Fusion: Tiny fast SSD does the boot up and swaps files in and out of the HDD (mechanical hard drive) running less than 1/20 the speed of the SSD. The Mac OS makes the decisions—the user has no independent control over the process.
Pros: Fast boot up. Cheap. If you browse the web, do word processing, email, watch YouTube videos etc., you would notice little or no difference between the two ever.
Cons: If you work with certain types of large files or you have filled up the drive, things get sloooooow. You have a mechanical motor generating heat. Beginning 2013, this was less of an issue when Apple started using slower, cooler HDDs. In 2015, the drives got even slower and ran cooler to increase reliability.
HDDs wear out but they can be replaced with SATA III SSDs (about 1/6 speed of NVMe 3 x4 blades). The best course is to just remove the HDD and upgrade the NVMe SSD to a larger one (2013-14; 2017-2019), larger and faster (2015)
SSDs: Beginning in 2013, Apple began using NVMe 2 blade type processors — look like RAM chips — in the PCIe bus for SSD storage. In 2015, the PCIe bus got faster to 3 x4 but the blade didn't in the iMac. In 2017, Apple began using NVMe 3 x4 blades and still do. Dirty little secret: Replacing the NVMe 2 blade with an NVMe 3 x4 in a 2015 iMac boosts it to near 2017 speeds.
Pros: Fast. Speed doesn't degrade. If processing large files on your boot drive, it can't be hampered by swapping out to a slow HDD.
Cons: Price. That's it.
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To rank performance:
1. SSD - best performance, costly, limited storage
2. SSHD or fusion drive - better performance, less costly, higher storage
3. HDD - great performance, cheaper, highest storage
I don't agree with 3. Mediocre performance — better than my G5.
My morning routine begins with rebooting my iMac and automatically opening the apps I need and logging into certain sites. I must start with a reboot or certain databases do not organize properly which takes time.
My 2010 iMac i7
Original WD Black HDD: When I was running from an HHD, I'd start up, have breakfast and coffee, return to the desk in 20 minutes to see if I was ready to work. Compared to my earlier G5, this was a big improvement.
Change out to 850 EVO SATA III SSD: Boot, get fidgety if the entire process took more than 90 seconds. Never took 2 minutes.
2017 iMac Pro
Process now takes 30–45 seconds.
A lot of the boot time is spent logging into web sites and the boot drive has little control over that.