Hi there,
Sorry to hear about the problem you are having. FPS drops while opening the tab bar is a problem that I have heard of quite a few times over time. Personally, I also had this problem. Thankfully, there is a solution that works for most players that experience this problem. Hoping this will help you, I invite you to follow these few steps:
• Go to your Steam library;
• Click on your ETS 2 game and select "play";
• Select "Launch with OpenGL" 64 bit;
• Drive a short distance in-game, then save your game;
• Launch TruckersMP
Hopefully this helps you!
Sincerely,
Expectancy