In your Options go to General Settings. Then under Traffic, you should see the screen where you can set either real online traffic, as well as the slider on how dense you want the sim to populate things like Ground aircraft, ground crews, cars and boats.
Scroll all the way down, and you should see the options for:
Use Generic Model for Multiplayer
Use Generic Model for AI Traffic
Turn these On, and both Multiplayer and AI traffic would be using your generic models. So no more custom model, no more custom liveries, no more stutters.