As described in the previous alpha builds, once the user changes the GUI Scale setting, the current session will be suffering a massive performance decrease until the client has been restarted. From what I observed this primarily affects the use of shader packs, ComplementaryReimagined_r5.2.2 + EuphoriaPatches_1.3.2 being used as an example however not exclusive to it. Also this happens only, I believe, when a shader pack is currently active during the settings change.