Export Audio Mixdown using external VST

MP3 could be suitable. It won’t give you quite the same fidelity as a .wav file. It depends a bit on what the audio material is. I would strongly suggest you bump up the bitrate of the MP3 to 320kbps.

As far as I understood it, you are not using any VSTi’s. (Virtual Studio Technology - instrument, i.e. a software synth.)

What would?
The length of your composition alone should not be a factor when it comes CPU utilization.

How optimized is your computer for handling real-time audio?
(Here’s a guide to get you started)