What I've done sometimes is cut and paste to a separate folder bgl files from the scenery folder in question until the FPS improves. It will mean certain scenery items will not be loaded but often enough they are minor and the bulk of the scenery will be shown. For simplicity I paste to the addon scenery title folder so I know where they belong if I want to replace them later. For example in the structure SCENERY A/scenery/texture, cut bgl files from scenery and paste them in SCENERY A folder. I had to do significant bgl cutting with addon KFCM as resizing didn't help at all, still looks good though.