it could be both, is there any part of the code path, that is creating/adding to something that might generate a very large memory requirement, such that after 30 seconds (i.e. 70 fps would be 2100 calls at the end of 30 seconds), so if your generating something, that is constantly consuming memory (yes it would have to be a lot), you could see a slow down in the blueprint, if you have a search function, etc of say an array, etc.
This is just guessing, as there is really not enough information to know, so just a thought.
Hope this helps,