lagging plug in handling

I have many problems with lagging plug in handling on my two studio Macs, one with WL9 Pro, one with WL9 EL. Both machines run OSX Yosemite. The problems are plug ins not reacting to clicking and moving on screen. Suddenly they react and then everything is stuck again. I haven’t checked all plug ins but several brands seem affected, yesterday it was horrible with Acon Digital DeFilter. All plug ins run fine otherwise, especially in other Steinberg programs like Nuendo 7 and Cubase 8.5.