I am very 50/50 about my otherwise beastly iMac Pro. The machine is very fast, you can tell, but Apple’s way of running software – and the OS itself – seem to cripple it. There were these investigations that even starting CLI programs leads to Apple system software phoning home – not sure if that’s true but you really can tell that something is going on and stuff isn’t running as smoothly as it can.
I am very disappointed. Even though I can’t exactly put my finger on it, I did expect much smoother operation from such a powerful machine. Seems like the actual problem is with Apple’s kernel and runtime.
So I can’t say with 100% certainty that the machine has slowed down – more like it never was as fast as I would have imagined such hardware should perform.
In the meantime, an old i3 CPU-based ultrabook laptop running Manjaro Linux outperformed my iMac Pro a few months ago, on a very parallel workflow that is more I/O bound than CPU (and it didn’t have an NMVe SSD like the iMac Pro has; its SSD was fairly regular average drive with 500MB/s read and write). This disappointed me and cemented my decision to eventually move to Linux forever and try my best to replicate the beautiful desktop environment that macOS has (although the main selling point are the Retina screens, not the UI itself which is pretty but fairly mediocre and not that functional).
All that being said, I really do use my iMac Pro a lot and have likely ~100,000,000 files on its SSD (I play with information back up quite a lot) and I suspect the OS was never designed for this so I am entertaining the idea of buying an external 2TB SSD and offloading all those files there – and disable indexing that drive from system settings – and see if that helps. Because it’s the only correlation that I could think of. 
Or I could just switch off Spotlight indexing and use Alfred. I feel I’ll become an expert in running macOS smoothly during the next months.