OMSI didn't boost when it comes to high computer specs, I recommend you install 4GB patch to see if it feels better. But from my own experience, it doesn't change a lot, It doesn't matter what specs your computer is, the game cannot handle itself very much due to lack of development. Or maybe you can lower the graphics settings to see if there are any improvements. For example, turn down the viewing distances, the AI-traffic numbers, the reflectional pixel, etc.
my computer i7-2700k, gt630, 8G ram, hong kong map fps is better than you. You should check any error in the log file. Error may reduce you fps. Missing ai car also reduce fps. Too many ai type may also reduce fps.