That's not really enough information to go on. Most importantly:
- What kind of i5 is it? The range covers everything from a 2014 dual core at 1.6GHz to a 2019 hex core at 4GHz+.
- What maps are you talking about? Grundorf and other simple maps are quite easy to run at 30, even 60+ FPS. Berlin or BotW, on the other hand, will cause even high end systems to struggle.
Then there are other factors too - what are your other system specs? Have you tried closing down all other programs before launching OMSI? What are your current settings?
However, all that being said, the most important things to try are:
- turning off stencil buffer effects and particle effects
- turning off "reduced multithreading"
- reducing AI traffic count to...