I usually run Linux, but recently got a 2018 MacBook Air and ran into this problem. I have a directory of just over 100 MB in around 200 files that I frequently backup and move between computers using USB-A flash drives. Using USB 3.0 on Linux it transfers in a few seconds, and the drive ejects immediately, in all cases.
When I first did this on my Mac, however, the same transfer was ridiculously slow (i.e. minutes) and I had trouble ejecting the drive, even when the computer was offline and nothing else was running. Every flash drive I tried did the same thing. Oddly, I also noticed that the flash drives seemed to work fine if they were already plugged in when I started or restarted the computer, but were slow when I plugged them in after the computer was booted up. There are a lot of other people online complaining about the same issues, so it's not just our particular computers.
After some research, though, I think I've solved the problem. The speed and ejecting issues seem to be mainly the result of two things:
The USB-A to C adapter you use. Make sure you get a USB 3.0 compatible adapter or hub (blue inside). I originally bought the Apple adapter, which is 2.0, and it took something like ten times longer to transfer that 100MB than with the 3.0 adapters I got later. (I don't understand why Apple wouldn't be selling a 3.0 adapter at this point.)
The Spotlighting feature in MacOS. I think this is the big culprit. Watching the Activity Monitor, it's clear a LOT of resources get eaten up trying to index the files on the flash when you do a large transfer, even after the files are all copied, and the drive won't eject until it's finished. The transfers are a LOT faster and the drive ejects right away if you turn off Spotlighting for the flash drive. Just go to: System Preferences -> Spotlight -> Privacy, and click + to add the drive to the opt-out list. (The drive will disappear from this list when you eject it, but the system remembers it by name and it will reappear there as an opt-out when you mount it again, so you only have to do this once for each flash drive. You might want to give each of your drives a unique name to track this.)
By using a 3.0 adapter and turning off Spotlighting, I'm now getting flash transfers that are more like the speeds I'd expect, and I'm not having problems ejecting the drive. (With Spotlighting on, starting or restarting the Mac with the drive plugged in seems to give the proper performance, as well, which probably means the indexing is taken care of during boot-up initialization.)
UPDATE: I also tried reformatting my flash drives to APFS format, and it sped up transfers even more - quite a bit more on larger drives. If you don't need to use non-Apple systems, it's probably worth changing them over to the new Apple file format. (Note - you have to change to GUID partitioning first, if it's not already setup that way.) Still keep the Spotlighting off - I think that's the bigger issue.