This is problem thats been around for a year or two.
I have a lot of albums in JRiver, and it works fine. The UI is smooth, all windows scroll cleanly, its a joy.
As soon as I play a track and the vst plugin starts, JRiver slows to a crawl, the windows won't scroll cleanly, the windows jerk and it just gets very unresponsive
Various people have brought the problem up over at the JRiver forum (for eg https://yabb.jriver.com/interact/index.php/topic,123589.msg856867.html#msg856867 ), and the author of JR suggests contacting you.
Are you aware of this issue?