im not much of an expert myself,but have you tried using debug mode to pinpoint the problem?
recently i struggled with framerate steadily dropping down. turned out that there was an event constantly spawning a new object,while being invisible,that dropped my framerates pretty fast.
although,i guess it could be anything really,ill let people with more experience than me try to solve this,since im still kinda noob myself lol.
also, posting a .cap here would help to solve this issue.