You have likely encountered a bug known to have existed in older versions of FMOD Studio. This bug causes projects to run more slowly the more assets they contain. It isn’t suddenly triggered by having more than a certain number of assets in a project; rather, it affects every project regardless of asset count, but is only noticeable in larger projects because the slowdown is proportional to the number of assets present.
This bug was eventually identified and fixed, and so is no longer present in the most recent versions of FMOD Studio.