I've reported this before and then after a beta update the problem vanished again, but now since the new update it's back.
When starting Rimworld the first time after starting steam, steam takes over a minute before actually opening rimworld. (After that it takes just a couple of seconds) It says it's synchronizing, but I have cloud sync turned off and there is no HD activity so there's no syncing going on at all. It just seems to be waiting for something. Other games don't have this behavior.
Hmm, I've never seen an issue like this. I guess it'd be a good idea to ask Steam support.