I’ve been seeing this since I updated my 4.9.0 install to 4.9.1 as well. Running 4.8.3 and 4.9.0 concurrently was no problem. A single instance of 4.9.1 will eventually pin my RAM usage at 100%. A second running instance does it in about 10 minutes.
Completely new C++ project, too, so I’m fairly confident it isn’t anything from a conversion process causing the issue. Blueprint could still be a culprit, though. I use it almost exclusively when sketching.
Edit: This actually appears to have been launcher related for me. Being vigilant about closing the launcher has curbed the issue entirely. YMMV, though.