Matters

So this is weird.
A complete uninstall including registry entries and Drivepool's program folder with Revo Uninstaller followed by fresh install with 2.2.5.1237 works with expected speeds.
Installing latest version 2.3.3.1505 over that goes back to 5mb/s.
A standard uninstall using Windows of 2.3.3.1505 and rolling back to 2.2.5.1237 continues to be 5 mb/s
Doing a complete uninstall with Revo and reinstalling 2.2.5.1237 again allows it resumes working at expected speeds.
I'm testing by using a folder with a 15gb video file and mixed smaller files and bouncing it between drives using a folder placement rule.  I also tested using a 500gb solid file. Only on a clean 2.2.5.1237 installation am I able to get expected speeds.
So it seems like at at some point something changed that negatively affects transfer speeds and persists unless you completely uninstall it from your system.
Anyway, going to try a different version next.

Edit: Tentatively 2.3.2.1493 seems to work well. I will update if anything changes.
Edit2: Nope, started noticing the same behavior. Worked my way back to 2.3.1.1448 and still getting slow speeds on re-balancing. It takes a really long time to reinstall, recheck drives, test balancing so I'm back to the latest version now as 2.2.5.1237 is missing a lot of the functionality of the later releases. I'm hoping a future update changes something. This is with unduplicated files. SDD to HDD balancing caps at 5mb/s and SDD to SDD balancing caps at 40 mb/s.
Edit3: Alright so I feel like an idiot but it looks like my SSD drives were running on a USB 2.0 interface and not a USB 3.1 despite it being in the same enclosures as my HDD which were all 3.1 per USBTreeView. I remember checking this before and the SSDs were running at 3.1 Superspeed. I think this is also why the 'fix' of swapping to a different Drivepool version stopped working because  at some point they downgrade to USB 2.0 and the transfer speed plummets. I don't think this has anything to do with Drivepool but wanted to throw this edit out there to save someone a bunch of misplaced effort. Anyway, not sure if its an enclosure issue (Mediasonic Probox, have used a handful and they have always worked well) or windows, but I reinstalled the drive drivers and reseated them so I'll see.
 
 

Sorry guys I wish I could help. It's been years since I made that post, and hitting the arrows button has always worked for me. There are only two things I can think of that may help.
One is that someone above said they've tried hitting the button "100x times". But it is a toggle, meaning that it is either on or off, so you only need to press it once, then wait. It can take a few minutes to have an effect, so don't press it again or you'll be turning it off.
The other thing is that every time I get a notification to update drivepool, I click on it but it fails for some reason. I don't remember the exact error message and I've never cared enough to figure out why because it works just fine, but what I'm getting at is that I'm running an older version of it, version 2.2.5.1237. Perhaps this is a problem introduced in newer versions.
If it's neither of those things, then I can't think of anything else. I hope you figure it out.