That's sadly not how it works. You can't just "bypass" an addon and quickly check how much memory it didn't use. It's a bit like throwing different ingredients into a bowl and stirring them - you can't just take out an egg after that.
You'd have to completely disable an addon, reload the UI and then somehow restore the exact same conditions you had before the reload, otherwise the memory consumption may be different due to other factors.
That aside, memory consumption does not directly correspond to FPS loss anyways. If you want to find out which addon is causing your game to stutter, you could give
ESO Profiler a try. Just wait until you encounter the issue, activate it for like 20 seconds and then export the captured data for inspection in the external viewer. That should hopefully give you a clue which addon is eating your frames.