Audition lag when editing long project in Waveform

What happens in Waveform view is that every time you make a change, the temp file pile is added to, up to the limit of the number of 'undos' you've set in Preferences. So if you set that to 5, for instance, that's the limit to the number of steps you can go back. Obviously on a longer, larger audio file the size of the temp file is going to grow pretty huge quite quickly, as it's saving all of the audio each time, which is what it has to do in order to allow for (as far as Audition is concerned) arbitrary changes to a file, which could be anywhere in it.

So yes, do exactly as SuiteSpot says, but also try reducing the number of available undo steps and see if that helps. And make sure that there's a lot of room available on your temp file drive - these can build up in size very quickly, and on a slow drive, the access times will inevitably grow at the same rate...