the cpu does most of the processing to decode and playback the media files, but the storage ssd/hdd needs to be fast enough for the cpu to read the media in response to playback or scrubbing demands. mp4 codec is for distribution, its not edit friendly as cpu's struggle to decode mp4. i would recommend transcoding or using proxies, using the cineform codec. cineform files are much easier for the computer to read and scrub. cineform files will be much larger than mp4, and can be more demanding on the storage. your 10 year old hdd probably isn't very fast compared to modern hdd's, so depending on your budget you could look at a new hdd or ssd to help out with the cineform files if needed.
Premiere Pro in a positive way demands performance from multiple PC components - CPU, RAM, and drives. So, balanced systems tend to work the best. Fortunately, or maybe unfortunately, your system is balanced but relatively slow compared with a new, powerful system.
That being the case, I'd venture to guess that you are bound most by your CPU (only 4 threads - quad core, no Hyperthreading) and next by your RAM. It's pretty easy to prove this out. Call up task manager / performance tab and see if your CPU load is max'ing out when you are scrubbing and noticing a performance hit.
Your drives are not too powerful but 1080p mp4 doesn't really require much bandwidth. Too, if you had 32GB of RAM, Premiere Pro would be using some and you'd have some excess capacity for Windows drive caching (using RAM) which does indeed help scrubbing performance.
so since posting this question I bought another Samsung 850 EVO to replace the old hard drive I was storing my conforming audio files on. I also bought a 960 PRO m.2 and installed fresh windows 10 on it. I am capped at 10 Gb/s because I'm on a z97 board so I not getting full speed out of the 960 PRO.
Those might have improved scrubbing a hair but barely noticeable. I tried installing a GTX 1070 over my 970 and didn't notice any performance gain. I went ahead and picked up the i7 4790k to replace my i5 4690k and as you surmised that made a pretty noticeable difference in scrubbing speed. However I've noticed 720p mp4s and dvd quality avi's will scrub perfectly silky smooth no matter how fast I move the bar but 1080p mp4s still have just a slight hesitation comparitvely. I really want to get perfectly silky smooth scrubbing.
I can try upgrading my ram to 32gig as you suggest but It does not appear I'm using more than 40% ram currently while premiere is open and I'm scrubbing.
I've read that kaby lake and sky lake aren't much of an improvement over hawswell/devils canyon but that's mostly coming from people who are gamers. I'm wondering if I upgraded to a z270 motherboard with either the sky lake or kaby lake i7 if that might get me to that perfectly smooth scrubbing in 1080p mp4's I'm looking for?
I don't mind spending the money if it will get me the silky smooth scrubbing I'm getting with 720p mp4's.
If I were faced with that motherboard I might try something else with your components.
- First I would not use that 960 Pro as a boot drive. It is wasting the speed because as you boot up everything important is really running memory. Use it for your project files as much as you can. Use your SATA III EVO for booting.
- While your motherboard M.2 connector is capped at 10 Gbits/s if you buy a $20-30 PCIe 3.0 adapter card and I would imaging that you will get full speed. It might cut you GPU back to x8 but that would be a small sacrifice