New
#11
Thanks for all replies. Anibor and muchomurka replies both support what is happening in my case. It seems to be a combination of both SSD and CPU thermal throttling. External SSDs, being compact, cannot dissipate the heat efficiently. The bottom line is that, in my case, using an external HDD is better than using an external SSD for copying 60GB image files to. The HDD speed is slower, but it is maintained with no drop-off in speed, resulting in faster task completion overall. This does not apply as much to internal SSDs since they have better heat dissipation.
I'll call this "Solved" and thank you all again.