What Video editors found is that the slowness of only one Rotating Magnetic drive, on which you stored ALL your files, DID slow down an otherwise speedy Mac. Too much time was spent waiting for that ONE drive.
When you store Source materials on one drive, Destination materials on a different drive, Scratch or Libraries on yet another Drive, and NONE of those other types on the Boot drive, you regain the speediness. Source files needed are read-ahead, and files being written are first written to a System RAM buffer, then copied to the drive when it becomes available.
When all video/photos are on ONE drive, the system waits for each group of Reads or each group of writes to complete before beginning another. When you have multiple drives, the system can overlap the I/O across the many drives and get your speed back again.
So for speedy operation you need MORE drives, not necessarily really fast SDD drives for everything.
Compared to the cost of the new computer, a few hundred more for additional drives is cheap.