Most of the issues I am noticing are in FCPX. Just slow, lots of beach balls, even after allowing files to input and process. Surprised that the 2013 rMBP would be snappier. I guess that is all in the SSD vs the Fusion?...
It likely has nothing to do with SSD vs Fusion, nor i5 vs i7. I have both SSD and Fusion iMacs and have tested them extensively on many benchmarks, including various FCPX tests. There generally isn't a huge real-world performance difference and certainly not due to disk I/O. I also have a 2015 top-spec MBP with 1TB SSD and it's no faster than my 2013 iMac 27 with 3TB Fusion Drive on FCPX tasks.
I have tested my i7 iMacs on FCPX by disabling hyperthreading with a special utility called "CPU Setter" and that only made about 30% difference and only on very specific tasks. An i5 would not account for a widespread general slowdown.
You should not be seeing lots of beach balls (officially called the "spinning wait cursor"). That is a sign something isn't right, not just a slow app or task. E.g, intermittent hardware failure, non-responsive network I/O, old non-compatible FCPX plugin, disk space exhausted, editing media on a USB 2.0 HDD or thumb drive, etc.
If you have any doubts about your Fusion Drive you can run the BlackMagic speed test and verify it is performing as expected:
https://itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550?mt=12
You can also run GeekBench to test general CPU performance:
https://itunes.apple.com/us/app/geekbench-3/id683676887?mt=8
If you suspect a hardware problem you can run Apple Diagnostics. This is not comprehensive but sometimes finds obvious problems:
https://support.apple.com/en-us/HT202731
Different codecs and resolutions can make a huge difference. E.g, H264 1080p on a MBP will be a lot faster than 4k on a top-spec iMac 27. Likewise some plugins like Neat Video noise reduction are very slow, esp. on 4k.
Since FCPX has seamless built-in proxy support you can usually get faster performance by transcoding to proxy and setting the viewer to proxy. You must remember to set it back to original before final export. However on most Macs (including yours) it should not be necessary to use proxy to get good performance on most H264 1080p content.
