It'd probably have to be addressed on the JRiver Media Center side with (my guess) some sort of workaround or compatibility setting added to the DLNA/UPnP settings, as I don't think the BubbleUPnP, USB Audio Player Pro, Neutron, etc. developers would be willing to implement workarounds for this issue as it only happens with JRiver Media Center. I suppose the logic on the app developer's side would be since it only exists with JRiver Media Center that it's ultimately a JRiver Media Center issue. To that effect I can say the slow loading of lists issue doesn't exist with foobar2000 with BubbleUPnP with the same library of files, though I don't like how lists are setup/formatted in foobar2000 vs. MC (which is much nicer) so I don't use that other than testing this issue.
And yes, the issue still exists even with me upgrading to 1 gig fiber with upgraded higher end networking and PC hardware for the server so I don't think the issue is there. It's something with MC and it only happens with very large libraries. So I get it's rare for anyone to encounter it and only a handful of us have reported it.
So yeah, hopefully something can be done on MC's side to make it play nice with all these apps, as that'd be the path to least resistance here versus trying to get the apps to workaround it, which likely won't succeed. Primarily I use JRemote and MO 4Media to stream to my phone, but the USB Audio Player Pro app in particular supports bit-perfect playback when using my iFi Go Link portable DAC. It even allows DSD (and MQA if I cared about that) to work through my phone with the DAC connected.