For instance, when I place a 60 second video onto my timeline in Final Cut, I have to sit there and wait for like 3 minutes until it fully renders. When I extract about 100 photos from Lightroom (10MB each), I could probably fold all my laundry by the time that's finished. Therefore my workflow is totally screwed. Is that what an SSD would fix or the CPU or GPU (don't know much about those)? Thanks so much for your input.
1. Lightroom image export is generally CPU bound. That said, 100 images shouldn't take THAT long unless you're using insane settings or working with insanely large (raw) files. I would try watching activity monitor the next time you do an export and see what the CPU usage looks like. If you're hitting close to 400% (for your iMac's 4 cores) then its most likely your CPU speed. That said, be aware that CPU speed hasn't increased THAT much in the last 6 years. Even if you buy a top of the line 4Ghz i7 model you're probably only looking at a 60-75% increase in speed.
2. I'm not an expert on Final Cut, but are you utilizing any effects? Final Cut is optimized to take advantage of GPU(s) to accelerate certain types of work so its certainly possible the old, slow GPU in your iMac is holding you back. Still I'm not sure if this applies to the timeline importation phase so someone with more experience can hopefully comment on this.
Also, have you considered that your hard drive might be (slowly) failing? That would explain why applications with IO access (importing a clip into your final cut timeline, exporting photos) are slowing down.
One more thing, is this something that has gotten worse over time, or has it always been this bad? If its gotten worse over time I would suspect some kind of hardware issue (probably failing HDD)
Regardless of the CPU/GPU limitations of your machine, I would still consider getting an SSD and seeing what that does for your performance before splurging on a new machine (unless you just want new and don't want to wait, which is also ok). It should certainly speed up almost every step of your workflow, and make your machine noticeably more responsive.
This post is a bit of a mess, I'll try and clean it up later if I have time. Please ask if there's anything you don't understand.