Making Mac Pro 5,1 cMP I/O Faster For FCPX

Greetings!

Working with 4K is frustratingly sluggish. For example, when:
- adding a transition and wanting to view it immediately without waiting several seconds for it to render.
- scrubbing in the timeline is not smooth.
- can’t play-back transcoded 4K in the timeline without dropped frames. Forced to use proxies.

A couple FCPX Notes:
I always transcode to ProRes on import.
I almost always export to H.264.
Most of my footage comes from the Sony A7III and Phantom 4 Pro.

I recently upgraded both CPU’s and GPU; noticed almost no real-world difference when editing.
I recently learned the SATA 2 bus in the cMP is a known I/O bottleneck.
So, I/O is where I’d appreciate some coaching, please.
My boot drive is a 500GB Crucial SSD.
My FCPX libraries live on a 6TB WDC HD.
Does anyone have any specific thoughts about what I need to do to make my Mac Pro chew through 4K effortlessly?
Do you think I need a/some NVMe SSD's in a PCIe Adaptor to speed things up? If so... which products?
Thanks very much!

Mid 2010 Mac Pro 5,1
Dual 3.33 GHz 6-Core Intel Xeon (X5680’s)
Sapphire Radeon Pulse RX 580
48GB > 6 x 8gb > 1333 MHz DDR3
OS X > I0.14.6 Mojave
Firmware > 144.0.0.0.0
Final Cut Pro 10.4.7

in its preferences FcpX has two check marks for" warning drop frames messages" ("general" or specifically hard drive related ) please thick both. and... what does the message say ? this is the first diagnosis

then you are reading 4K from a single Hard Drive ? You need at least two of those babies RAIDED , mandatory

aside from the speed of the hard drive it should not be that sluggish with a RX 580, specially if Optimised

Thank you for your help, Xavier Novembre.

RAID... yes, agreed. I'm currently shopping for PCIe cards, NVMe SSD's, and considering the use of RAID 0.

When importing optimized 3840x2160 from a Sony A7III, for example, I get the attached error message:

you would need to RAID "classical HD" (the best bang for the buck) or use SSD (no need to RAID them for playback) with limited amount capacity of storage

start from there and see how it goes.

the message is not HD related though. Strange because the RX580 should be fine.

Thank you once again, Xavier Novembre.