Unfortunately, I am having this exact same problem. I am on 13.1-beta-2. The irony of this is I was watching the developer fix this in a live stream from 2016 today. He was going on about Windows Forms vs WPF and caching. I was like... my LaunchBox build never ran that smoothly, and I have been using this software since 2017. It drives me crazy, but I have learned to live with it.
I have also tried the recommended remedies mentioned earlier in the thread, and nothing improved the performance.