How to speed up file transfers to flash drives?

Many users tend to have old flash drives that are slow and often choke on large sustained writes.

I have such old 128 GB SanDisk that works OK when writing a few smallish files to it. But when writing large files, its cache is soon filled and it gets VERY slow.

On the other hand, I have a 1 TB SSD OWC Envoy Pro Mini which uses fast SLC for the whole 1 TB storage so it writes those large files without hiccups.

Other SSDs might use SLC only as a small fast cache, and after it is saturated, they resort to slower MLC, TLC or even QLC which might drop to HDD speeds.

https://www.howtogeek.com/444787/multi-layer-ssds-what-are-slc-mlc-tlc-qlc-and-mlc/

The "speed" at which you can copy or move files from your Mac to a flashdrive will be based of a number of factors:

• The "quality" of the flashdrive that you use.
• The data rate supported by the drive.
• The data rate supported by your Mac.

You have provided very little information in your post. All we know is that you have some 27" iMac model that is running macOS Sequoia 15.5, and that you use flashdrives. Not a lot of information for us to go on to assist you with this.