Sensuki
Arcane
Bring up Task Manager, leave it up.
Run PoE. Get in game. Make a note of the memory usage in Task Manager for PoE. (If you have a second monitor, putting the Task Manager on that second monitor can help a lot).
Wait for a period of time without doing anything in game.
Check the memory usage again in Task Manager and compare it to what it was before.
If the memory is substantially higher the second time, there may be a leak.
I found exactly what causes the FPS drop, it's when you move a character in the formation menu. The FPS drops and never comes back. It's not a memory leak.