My frames were ok (around 40 fps in dense areas) but every 3-5 sec the sim was like frozen for a second or so. After trying a lot with graphic and data settings I found out that my problem was network related. My Mobo has two LAN connectors which were connected to the router via a network bridge. I erased that network bridge and separated the two connections and my stutters are gone.
Greetings Uli