What changed in FCPX 10.4 that makes it slower?

I hope this is just me because that means it's fixable. Seems the past few months my iMac is scrambling to keep up with FCP. It's gotten so bad that to view a draft video, I'll export just to view it without FCP stopping/starting every few seconds. I've removed from the background everything that's not essential from running. My files are on my local drive not ext HDD. I thought it was the rendering but the background checker (circle with checkmark) is complete and it still thrashes. No change to my HW otherwise, again, it used to flow like butter but something changed and it is super slow now. Cut the film under edit down to a short 8 mins, and that seemed to have no effect. About to plop down $600 for a 4TB SSD to replace my onboard HD. Have googled and checked around, and no obvious answers. 

Inspector shows all clips have original, optimized, and proxy versions rendered.

Much appreciated. This is killing my productivity.  

What are the system hardware specs? How big is the system drive? How much free space?

Clear the render files from the Library.
Be sure your system drive has at least 15-20 percent of its capacity left as free space.

Render files were deleted not too long ago as a remedy but did not effect it. I'm wondering if it might be some motion templates. So, I dragged several out of that movies subfolder to another one on my desktop. Great. But, as soon as I fire FCP back up, I can see that it auto-populates them right back in to the movies/motion templates subfolders. How do I stop that, so this eval can continue to see if that may be the root issue? No tips found on Google.

BTW, FCP is t he only app thrashing. Everything else on this iMac is super smooth.

Please give the complete system information or post an EtreCheck report.

Motion Template effects, if moved out of the Motion Templates, do not reload. They only appear in FCP if they are embedded in a project.

If plugins are handled by 3rd party apps like fxFactory they are reloaded every time you launch FCP. To remove them you have to do that in the "hosting app" in my example fcFactory, otherwise they are downloaded and installed each time you start FCP.
Some people do setup their FCP in way that 3rd party plugins are stored in the Library: never do that!!!!
Sooner or later this will screw up things seriously.

So check for "Who is responsible for my plugins"