If you could take a sample and send it to omnigraffle@omnigroup.com, that could help us figure out if it’s something specific to your documents or setup.
I dunno… It works fine and dandy on my Macbook pro 2014 edition with 16GB of ram (and a lot of apps running). I had an older version of Omnigraffle that worked fine as well, so this latest 7.xx is an upgrade to me on OSX 10.13.3
Runs faster than older version. You might check your OSX/console and see what kind of errors or wait spins you are getting
I am experiencing extreme slowness on my 2018 MacBook Pro, 2.6GH i7, 32GB RAM. However, on my old mid-2010 MacBook Pro with 16GB RAM, Omnigraffle works fine. I suspect the problem has something to do with the touch-bar.