You're hitting the limits of your machine as currently configured. Can you improve it? Maybe, if your configuration is not optimized. Read the audio optimization tips you'll find at Cakewalk's website or in the manual/help system.
If you have nothing left to improve there you'll have to start thinking about premixing some of your tracks. For example, mix four tracks down to one or two and deete the original four. Not ideal but it does allow you to keep going.
Are you using effects on these tracks? You might be able to improve things by assigning effects to aux busses rather than individual tracks, or by applying the effect "destructively" in so-called offline mode.
I hope some of these ideas help...