Urbackup very slow on specific file types

Hi,
Marvelous toolset BTW!!

I’m backing up a number of Win 10 clients (Urbackup 2.3.4) to a Debian 9 server (Urbackup 2.3.8) with BTRFS attached RAID array.

I’m finding several Windows file-types that slow that backup transfer to a crawl;

.one
.onebin
.bin

These are accossiated with OneNote files.

If I exclude .one files from the backup a typical machine will take 40 minutes to run an incremental backup.

With .one files incuded that incremental backup can report that it will take days or even weeks to complete.
The .one files in question are approx 100M in size and there are only a few of them.
Typically, the backup will fail as the client appears to go “offline” - though the client is still online and active.
During the transfer of the .one files the data rates fall to 10’s of KBits/s or even Bits/s shown in the Log file.

Can you explain this, or have I incorrectly configured urbackup?

Regards,

Steve

P.S. I see similar effects with OneNote .bin files (AppData/Local/Microsoft/OneNote/16.0/cache/*.bin)