Without knowing your exact settings in Twixtor and other things nobody can tell you much. The simple assumptions would be that
- your footage needs to be converted before bringing it into AE
- you are completely misconfiguring your render settings and preferences
- you do not really understand how Twixtor works and probably using hopelessly over-cranked settings, requiring it to analyse awful lots of frames just to synthesize a new one, yet only producing garbage
- GPU acceleration in Twixtor is not working right
Everything else we cannot know...
Depending on the settings Twixtor can be extremely slow. If you have a lot of rolling shutter artifacts or motion blur in your original video you can also get terrible artifacts. Here's an important thing. MP rendering most likely will not work with Twixtor or any temporal effects so turn it off. The only way to know if your machine is behaving normally is to look at the shot and do some testing. Some shots just require a lot of tweaking to significantly slow down.
If you shot at 100 fps and you want to playback in slow motion then the easiest way, and the best way to start is to go to Footage Interpretation and set the frame rate of your footage to 29.97 or 24 or what ever your deliverable product will be, then drop the footage in a new comp with the same frame rate. No Twixtor required.
Now if you want to slow things down more or speed them up you are working from a much better place and the whole process will be much easier and faster to render. Twitter will also work better.
Your footage should be an uncompressed frame-based video format before you get to AE. As the boys have noted. AE requires so much processing you just bog it down by telling it to also decompress or transcode. Do that externally even though AE can, if asked, do some of it for you.
There are literally millions of people using gopro-specificfootage smoothing apps from many publishers. You could try asking other gopro users for advice on how to smooth out the footage if you don't want to just use AE's built-in systems which have some drawbacks. But you'll find all smoothing apps require careful study and tweaking and they're all kinda slow
Twixtor has its own support community. It's a processing nightmare, always has been; it's got a ton of stuff to do and to track and remember and render.