Thanks for the quick reply!
I actually tried already removing just the background blur-effect and it didn't make any difference. .NET framework is up to date and so is Windows 10 (well I don't have the latest creators fall update yet). I've also tried the usual general performance tweaks from here: https://forums.launchbox-app.com/topic/35368-bigbox-performance-and-how-to-make-it-better/ , tried removing all transition animations etc. My system is relatively good: 2500k @ 4,4Ghz, gtx 1070 so it shouldn't be the problem. Also, I have less than 30 roms and only 3 platforms so that isn't the problem either.
User Ripklawe seems to have similar problem in this thread: https://forums.launchbox-app.com/topic/39319-background-images-question/ (If you read the whole short thread) Though he seems to have similar problem with every custom theme. I've tried CleanBG theme with rather similar gameview (with game dependent background) and it doesn't have any performance issues and as I said the two default themes don't have either.
This isn't a huge issue because the theme is really nice looking even with white background, but it would be interesting to find the culprit. Cheers!