I have 2 External SSD for the iMac M3 24gb 8/10 core. Both of them starts fast then slows down then speeds up again then slows down to a crawl. I transferred 300gb of data to each one. One at a time to test. It starts off super fast...then about 1/4 or 1/2 way it starts the fast/slow/fast/slow/ action... so this is caused by Mac OS 15.2? Using OWC Thunderbolt 4 cables.
Did this just start recently (e.g. with upgrade to macOS 15/15.2) or has it been like that since you got the machine/these SSD? What make and model of external SSD?
Which filesystem (APFS, HFS+, exFAT) are you using on these SSD? Are they APFS (Encrypted)?
How full are your drives?
Note that it is normal (expected even if not ideal) for modern consumer SSD to go fast for the first the ~ 50GB and then slow down dramatically after that. Scroll down to "Sustained Write Performance and Cache Recovery": on this page:
The WD Black SN850X is one of the fastest drives around for gaming.
www.tomshardware.com
As such when I look for an SSD, I specifically search for those with high sustained performance (after prioritizing high TBW and low peak power draw) rather than peak performance. For external TB3/etc drives, performance above 3GB/sec is inaccessibly anyway so I don't care about that. But thermal throttling, non-cached slowdown, and especially drive failure will slow me down.
In any case, if your issue didn't start with macOS 15/15.2 and/or you aren't using APFS (Encrypted) it is probably unrelated to this thread and probably belongs in another thread. If you do repost in another thread still include the above information as well as your actual performance (e.g. it transfers the first ~ 25% in 25 seconds but then the remaining 75% takes 5 minutes and/or include results from one of the test programs listed in the links to other articles).