I edit at work on a new Mac Pro model - 8 Core, 3.0 Ghz, 32 GB of memory, 1TB Flash storage, Dual AMD Firepro D700. Media is placed on a external RAID 5 connected via thunderbolt. Most of the time I don't get a "red" render bar on top, especially if I'm matching the media with project settings. But even if I'm cutting Canon, Sony, or RED r3d files and previewing for Prores, most of it can be in real time and at most I get a "yellow" bar and pretty decent playback on 1/2 to full resolution. "Red" bars come into play when I'm adding color, effects, etc. or heavy multi-cam cuts. And when I do have to render the "red" bar for previewing, exports, etc. it goes pretty quick.
Now I just purchased a Dell Precision 7910 and did some upgrades (memory,etc) - Dual 8 core Xeon, 2.4 GHz Turbo, 32GB of memory, 480GB SSD boot drive with internal RAID 0 created through BIOS for media. Graphics card is Nvidia NVS 310, 1GB. I'm going to be swapping out the NVS in the next month or so for a GeForce 1070. Now with the current PC setup (without new graphics card), I get a "red" bar for everything that isn't matched (media & project settings). And when I render it takes several hours.
My question is, what are the biggest factors that are creating this gap in performance? Is it the graphics card? I'm new to PC so I don't know if I'm overlooking something. I do a lot of cutting with r3d 5k, etc. and haven't had any real issues on the Mac. I'd like to get the PC to that level.