OK that's nVidia. Normally it doesn't make that much difference as you can see between the 780m and M395X I tested. However in a few cases one will be much faster than the other for a specific task. It doesn't mean one is generally superior but that's just how software and hardware works.
A few items:
- In general you don't need to render the timeline in FCPX when editing. You can usually turn off background rendering. If you are waiting on a background render, you may be waiting unnecessarily.
- Some people use the term "render" to mean exporting to a file. Technically this is encoding. If you perceive that phase is slow, output to single-pass H.264 so it will use Quick Sync, which is Intel's hardware-accelerated encoding. In FCPX: Share>Master File>Settings and use:
Video Codec: H.264 Faster Encode
Resolution: 1080p or 720p
For uploading to Youtube or Vimeo, 720p is often sufficient.
Each effect (esp. 3rd party effects) have their own unique performance characteristics. Some are pretty quick and some are incredibly slow. Sometimes there is justification for the slowness and other times it is just poorly written or optimized.
The problem with poorly optimized software is you can only get a machine so fast. E.g, on my iMac I have both Premiere CC and FCPX. Exporting to H264 Premiere is about 4x slower. Fast forwarding on an H264 4k timeline Premiere is 20 times slower. It's impossible to buy a machine 20 times faster -- it must be addressed in software (which I believe Adobe is).
mTitle3D may be uniquely different between the MBP and iMac for unknown reasons. This doesn't mean the iMac is a poor machine. A MacBook Pro can often export to H264 faster than a 12-core Mac Pro because the Xeon CPUs in the Mac Pro don't have Quick Sync. It doesn't mean the Mac Pro is a bad machine -- it has other very compelling qualities which is why so many creative professionals use them. If I could afford a 12-core Mac Pro I'd have that instead of a top-spec iMac 27, but I am happy with the iMac.
That said if you are generally unhappy with the iMac and can live until 4Q this year you could return it then optimistically hope Apple will greatly upgrade the GPU. Even then there is no guarantee that will happen or if it happens whether the degree of improvement will solve the issue with mTitle3D.