[throwaway48476]

This has nothing to do with heat. Almost all SSD's have an SLC mode where part of the drive is operated in fast SLC mode. After the SLC cache is filled it will write to the slower TLC/QLC area. Any SSD benchmark will show this. SMR hard drives also do this.

[jerf]

SSDs I know about, but is that true of every USB flash drive? Or any?

And it definitely sometimes has to do with heat, or I couldn't speed my drive up on-the-fly by cooling it.

[throwaway48476]

Only the controller throttles from heat, not the NAND. Sometimes they're co packaged in USB drives.

[nothercastle]

That’s probably what happens. That’s why they go from 120mb/sec down to a sad 20