Resilio Sync writes to ShellExtIO.log for every single file you view in File Explorer. C:\Users\<username>\Appdata\Roaming\Resilio Sync\ShellExtIO.log
Unbeknownst to me, this was wreaking havoc on my machine, triggering virus scanning and slowing everything down. I finally, after quite a bit of troubleshooting using ProcMon, figured out that it was these writes to ShellExtIO.log that were killing my PC performance.
This issue seems to be exacerbated on folders which are git repositories due to some bad interaction between Resilio Sync and Tortoise Git. Even without navigating to the subfolders, they would get logged in ShellExtIO.log.
To resolve this, I denied all write permissions to ShellExtIO.log, but this is crazy! I shouldn't have to do this. This took me forever to figure out and cost me a ton of time. This has been reported so many times, this needs to be fixed: https://forum.resilio.com/topic/42279-what-is-shellextiolog-cant-delete-it/ https://forum.resilio.com/topic/42564-shellextiolog-growing-out-of-control/ https://forum.resilio.com/topic/51435-bittorent-sync-uninstalled-for-months-but-shellextiolog-continues-to-grow/ https://forum.resilio.com/topic/43689-resilio-sync-is-killing-my-disk/ https://forum.resilio.com/topic/42362-bittorrent-sync-logging-every-single-filename-i-see/ https://superuser.com/questions/1164376/out-of-control-shellextio-log-file-from-bitorrent-sync