CPU issues with UAD plugins

Hello, I'm trying to use Mainstage 3 with my Apollo 8 and UAD plugins.

When I insert a UAD plug, my Mainstage CPU meter still charges. How come ?

Two other issues :

- When I insert the Lexicon 224 for example it adds 26ms, is that normal ?

- My CPU makes jumps and cuts audio once in a while with minor plugin configuration, it doesn't do it if I setup the same configuration in Ableton Live for example, with the same buffer settings, anything I'm missing ?

Thanks a lot

Matthieu