This is to be expected. You are using an experimental feature that alternates the rendering engine drastically and as such you should expect an FPS drop. This should be marked as invalid since this is not a bug but simply because using a filter acts like turning up the graphics thus taking up more RAM and processing power and lowering your FPS.
This is a mild inconvenience. Just for compensations sake; confirmed.
Hopefully something might be done about this, but I don't think many people are placing signs with text on them the way you are so I don't think it would affect many people. Besides, BdoubleO100 would be sad if his cool floor got ruined.