OK so when I add the effect card wipe to a layer in my composition, everything slows down, stops playing in "real time" and can barely scrub. Works fine when I take off the effect.
Uh-huh. New to AE, huh? That'll happen with some effects. If I was you, I'd spend some quality time here:
Care Wipe can be a resource hog. There is no way to tell if the behavior is normal without complete workflow details including the size of your comp, the fame rate, the structure, your system specs and version of your OS and Adobe apps down to the last decimal point.
If you are working with standard production footage in standard (not custom) comp size and frame rate, and you have a properly setup and current version of AE running then everything is probably normal for that effect. It you are throwing anything else at the project expect it to slow down. Many of my final composites can take up to a couple of minutes a frame to render at full resolution. That's the way compositing goes.
What the others said - you just need to get used to the fact that not everything in AE is realtime.