I'm not with Laminar Research, just a fellow simmer in the UK, but as I understand it, you will no longer be able to download X-Plane 11.26 from LR, as only 11.4rc2 is now available.
If however you managed to save a copy of your entire X-Plane 11.26 file structure on your PC before updating to 11.30, then you could always roll back to 11.26 by swapping one file structure for the other.
Personally I have used XP11.30 for a while and now use 11.4rc2 and have never experienced a major issue with frame rates @ 30-40 fps on average. Yes I get the occasional drop in fps for a moment or two, but normally this resolves itself after a few seconds.
Frame rates are dependent on many things such as hardware specification, XP11 settings, the type scenery being used during flight, internal/external views during flight etc.
Have a look at the video here :- https://www.youtube.com/watch?v=mljKJnZeALM which was prepared by the well simmer Q8Pilot and deals with the issues of XP11 settings in general.
As far as hardware is concerned, just check here :- https://www.x-plane.com/kb/x-plane-11-system-requirements/ to make sure all is OK?