Premiere Pro performance suddenly slowed down to be virtually unusable, even for simple projects

Question

I'm running latest Premiere Pro update on a late 2013, 27" iMac, with a 3.4 GHz Intel Core i5 processor with 8GB RAM, Mac osX version 10.9.5.  I know that's not a lot of RAM, but up until the past week or so I was able to edit an hour-long 4K project with that configuration, as long as I ran the Premiere session at 1/8 resolution.  It did drop some frames if I tried to play a long piece of it without stopping, but it was workable for editing purposes.  But in the past few days, the session has slowed down to the point where it starts serious frame dropping from the moment I push play, and is basically unusable.   It also drops audio, and I can hear the drives clattering frantically as soon as I hit play.   My media is on 2-TB G-drives connected via USB3.
What I've tried so far...
- Ran diagnostics on the drives, and it found no errors.
- Tried linking the project to the identical files on the backup drive.  Same problem. 
- I made sure my Mac's energy saver settings were set so that the drives never go to sleep.
- I deleted all the media caches and Premiere preview files I could find.
- I reset the Spotlight privacy settings so that they do not index any external drives, and only a few folders on the internal drive.
- I made sure I'm set to devote the maximum amount of memory (6.5 GB) to Premiere.
- Finally updated my Premiere software to the latest version.
None of these things made any difference. 
- Then I thought maybe the session had just gotten too big and bloated for my system to handle.   So I made a new project and imported a simplified version of just one sequence, which only references some of my color-corrected video files.  That way I only have to have one external drive connected, and the session is referencing much smaller files.  Still no improvement.
- So I tried an experiment where I made a brand new Premiere project and sequence.  I imported just 5 video files into it.  The largest file was less than 3GB.  (I do Protools editing on that computer with files much bigger than that, on the same type of extermal drives, with no issues at all.)  With only a single 5-second video clip (and no audio) in the timeline, it still starts dropping frames after about 2 seconds.  If I put another 5-second clip in the timeline, the frame dropping gets twice as bad.  If I try to put a video crossfade on them, playback gets so jerky it's unwatchable.
So basically, I started out being able to edit a 60-minute long 4K video project with video transitions and multiple tracks of audio and video.  (It wasn't perfect, but it was workable.)  And now suddenly, I can't play a brand new session that has 5 small video clips in it, and no audio.    So it's not just my project or sequences that are the problem.  Premiere just no longer seems to be able to access and play back my media with any kind of speed.
What am I missing?  Can you give me some additional suggestions to try that I haven't already mentioned above?  (And yes, I know a lot of you guys probably consider my hardware setup completely insufficient for what I'm doing, and you're probably right.  But it was working well enough for me up to this point.  I'm so close to the end of my project, and I'm really just trying to get back to the functionality I had a week ago, so I can finish.)
I'd appreciate any suggestions.  I feel like I'm running out of things to try.

Answer 1

After a lot of troubleshooting, it appears the problem is related to the color corrected version of my video not being able to play back fast enough.  While my system had no issue with the raw 4K camera files, the increased color depth of color corrected Pro Res versions are just too much for my drives to play back in real time, even at low resolution.   Unless I want to do considerable system upgrades (which I don't), the solution is to keep my color corrected and non-color corrected video stacked on top of each other in the timeline, and to deactivate the video track containing the color corrected ones.  Of course, I can turn the color corrected track on when I need to check something with it, but for playback functionality while editing, I'm limited to the raw camera files. 
Obviously this is an imperfect solution.  But because I'm basically done editing and just need to do some title work and a little trimming, I can finish the project working that way. 
Incidentally, tech support suggested major upgrades to my memory and video cards.  But if I were going to attempt to troubleshoot this further, I would upgrade to a better RAID system before I would do additional upgrades to my mac.  It really feels more likely that's the primary issue here.