Foremost, slow USB write/copy speeds seem to be a hackneyed topic. But this issue is one I've not seen presented elsewhere. Thanks for showing this novice your support.
I'm copying the same .iso file to batches of 18 daisy-chained 2.0 USB devices via UNetbootin. I format and partition with Gparted; setting them up to Fat32. Sometimes the transfer process takes 30 minutes, while subsequent batches take up to 2.5 hours. Assuming it'd help, I made a copy of the .iso file so that all 18 USBs aren't pulling from the same file. I also reboot before performing each "batch".
What's the reason for these speed inconsistencies? Is there a cache or swapping resolution that can keep the speeds at a constant 30 mins?