The problem is that at least version 1.1 had a slight issue with memory leaks. It grew to consume more and more RAM. So I recommend to close and reopen it before doing the encode. Or you just close it and do the encode with the command line program openshot-renderer! That also means you can easily edit the video on one computer and then encode it on some other computer, e.g. a headless server. And Openshot encodes pretty quickly as well. Even on my slow old notebook I can export from FullHD in a very short amount of time!
Video editing has never been this easy, fun and fast!