While you CAN set weather to CLEAR for missions you generate you may not have that luxury with pre-built single missions or campaign missions.
Another factor is some mods are built to higher system requirments than the TW stock items. (Aircraft, terrain, ground objects, etc.) I often find mods tend to reduce my FPS. This is NOT a knock against the modders. They produce some high quality stuff but you pay for quality in system resources.
The other issue you may be having with ODS is that the campaign and single missions are very busy (lots of objects and other missions). I find this drags down my FPS. I have been experimenting with a "lite" campaign definition that reduces the number of missions generated but it needs more testing and I will need permission from the ODS team before posting it. If I post it it will be at combatace.com.