Slow performance Mac Pro

Question

I've noticed recently my Mac Pro loads very slowly, i.e. final cut pro, photoshop, handbrakes, lightroom, etc. Even when clicking on Open, Save, Close, etc, it's not instantaneous as it was before. I've downloaded Onyx and use it to clean out the cache. Restarted the computer and it's back to its normal fast loading. But after using it for a couple of hours, it slows down again. And again, I need to clear out the cache with Onyx, restart before it returns to running normal for a few hours.
I am not sure what is wrong and how I can correct this so it stays permanently fast and not having to rely on Onyx every few hours?

Answer 1

Are you seeing any PageOuts in Activity Monitor? That is another indication you have way too little RAM.
You appear to have made the most important adjustment -- separating your User files off the System/Applications/Paging Drive.
How did you accumulate 240 GB of Applications? My Mac Pro uses less than 17 GB for System and Applications, including the page file. (Admittedly I do not have CS-anything or Final Cut Pro.)
When you edit, are you storing source and destination on the same drive? Doing that forces reads to compete with writes and causes endless re-positioning of the drive heads. Each seek costs you an additional 10 to 20 milliseconds ( 0.010 to 0.020 seconds) of time wasted, which can quickly add up. Editing from one drive to another is much faster.
When you edit, are you actually dipping into Scratch? Scratch is an "extender" that allows you to edit stuff that is way too big for your system, but s-l-o-w-l-y. Using it in any substantial amount is an indication you have too little RAM.