Expressions that slow down the animation process usually contain recursive math. This is not a scripting problem, it's a problem with the way the expression is written. Without seeing the exact expression that you are dealing with it's very hard to say if this is the cause of the slow down.
You could also be running into cache / expression errors. We can't know without workflow details and seeing your expression.
Check out this thread and see if that helps: Duik expressions slowing down AE CC - MoGraph Central - ::: mograph :::
Did Rick's response help you out?
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Yes, i disabled cache, and it's better. I'm using Rubberhose and/or Duik and Joystick 'n sliders for animating characters. So with 2 characters in a same scene, it's very slow.
Without the cache it's faster, so when i have simple expressions, for bouncing or other stuff, i active the cache, and when i make people i disenabled that.
One thing will be cool on the next upgrade will be to not have to do that between different project !
Thanks for the response.