I fixed my issue by disconnecting my lettered network drives.
I'll instead access my network drives directly by the network path.
This may be an issue with Windows more than JRiver.
I changed my network topology 8 months ago and my main workstation slowed to a crawl frequently, especially when I'd be invoking Explorer for 'save as' or 'open'.
See, I don't use Windows Explorer, I use Directory Opus for most navigation so I never even thought about this.
Turns out, I had a network drive on a now non existent IP in Explorer's 'quick access'. Not even mounted. Just a simple link in quick access. Absolutely tanked performance on the system. Every time Windows Explorer was invoked, it would begin a futile attempt to poll that IP and take a solid 10-15 seconds of 'hang' on the new Explorer window to sort itself out and move on. Clearing out everything in quick access fixed it.
TL;DR, Windows is weird with network resources that you've put in Quick Access.
SMB is honestly still the 'best' easy way to access your network resources but using the paths is WAY more fault tolerant and more performant in my experience.