cheap SSDs lie about their write-speeds
I have a cheap Crucial BX500 SSD for my backups. It has a write speed of like 100-300MB per second, idr. But it actually is much slower than that.
When I'm writing a lot of files/data to it, the initially transfer VERY fast. but then it slows down to a crawl (by modern standards) of like 5-15MB per second.
I remember reading that these drives have a cache. When you copy files to them, they initially get stored in a very fast cache, and then it writes to the long-term-storage FROM that cache. The cache isn't very big, so it gets filled up quickly. And then the rest of the files can only copy over to the drive as fast as the cache gets cleared - i.e. as fast as the drive can actually write to its long-term storage.
Its not that big of a deal. Modern tech is still crazy fast compared to anything I grew up with, even with this annoying thing. I think the cache-before-write is a cool feature & probably super helpful in a lot of cases. But it does make for some misleading marketing about the true write speeds.
Also. Don't buy the cheapest drive. Get a Crucial MX or something. You want your drive to be reliable, and its worth a few extra dollars unless you're dirt poor and literally can't afford it (hence why I got the cheaper BX).
More expensive drives my have this cache-problem too. Idunno. I don't have a higher quality external drive, because of the aforementioned poverty.
(sidenote: Modern poverty is umm ... kinda shitty but still there's much to be grateful for. I have a laptop & internet & an external backup drive & food stamps & medicaid. For some other people, modern poverty is much worse. I'm fortunate to have a family with some wealth and a house to live in as a result, and a mom who's paying the bills since I'm disabled. Many others living in poverty are not so fortunate, and poverty in less-developed nations may be worse.)
P.S. About 16GB copied over to my external backup drive while I wrote this post. It was like 12 minutes. I'm not really mad at that. I just wish the marketing were a bit more honest.