AE isn't an editing application. It doesn't work like one. You need to get used to Previewing. Go to AE Help and look up the topic. And whatever you do, keep them fingers off the space bar!
The spacebar is fine to use in CC 2015 because they changed the preview mechanism entirely. all about previews in After Effects CC 2015 (13.5)
That being said, CC 2015 has a few bugs due to it being the first step in a complete re-architecture of AE's main internals, so I'd recommend trying CC 2014 for a bit: How to Find and Install Previous Version of Adobe Apps in CC 2015
And, in that case, Dave is right; don't use the spacebar! If you're new to AE, start here: Getting started with After Effects
Thanks for the answers - looks like half the stuff I do is more suited to Premiere Pro so hopefully that will work faster for the editing - and like you said, I'll try an earlier version for the AE.
If you look at the Info panel you will see that it is mixing audio for preview. If you user pad0 instead it skips this. If you hit space it seems to need to mix ALL the audio first. Maybe try soloing audio and loading it all into ram first.
I have space set to stop at stop point, so it works like a normal editor, and pad0 set like it used to be. Wish i could save ram preview still though!