Yes the issue is the H264 encoding and the decoding takes a significant amount of processing and time which makes scrubbing a problem. I would suggest converting the media to DNxHR which should play back much easier on the system like R3D media.
...I believe I have heard Eric recommend here using at least a 4GB of video memory GPU for working with 4K.....something to do with "frame buffering". That 750ti looks a little weak to me for 4K,but, Eric would know far better than me about your exact situation ! Overclocking the CPU to around 4.5Ghz is another thing that many users of PPro do to improve performance. The DNxR surely will help.
...In addition, drive speed and quality also affect performance. Though you mentioned two separate SSDs, you didn't say exactly what they were and how large they are. If you are using multiple video tracks of 4K , at a certain point you would need for the SSDs to be fast. Some SSDs have inferior controllers and do not perform as well with "incompressible data" which highly compressed digital video IS !!
A general suggestion for a system like yours would be to use one quality SSD for OS, programs, and Windows page file ONLY . This SSD would be a Samsung 850Pro series of at least 256 GB or more, ( 128GB SSDs do not perform as well). "EVO" SSDs are not recommended. Crucial MX 200 series SSDs are almost as good as the Samsung 850Pro models,but, are cheaper. The Crucial and the Samsung Pro SSDs both have very good controllers on them for use with video.
For BEST peformance, ALL your other files can go on a new Samsung 950 Pro PCI SSD which has a read speed of ober 2GB per second and a write speed of over 1.5 GB per second. Another option to improve the speed of your "media drive" would be to create a " RAID 0 " with 2 or more SSDs off the motherboard. In any case, you want your media and project files to be on a FAST drive.....traditional HDDs or other SSDs can be used as a backup ,or, archive.
ECBowen - Thanks for the response, I've actually used Cineform in the past with good success, I'd just prefer to skip the encoding process if I could.
JFPhoton - I was kind of curious about the GFX card, as it seemed people were often using a beefier card than me. It looks like the GTX 970 might be a good bang for the buck choice...
I haven't done much looking into overclocking, that may be something I'll need to investigate.
Based on your recommendations, my SSD setup might be a little weak as well...
OS Drive (OS, Propgrams, and Page Files Only): Crucial MX100 SATA SSD 256GB
Scratch Drive: Crucial MX100 SATA SSD 256GB
Video Export Drive: Crucial MX100 SATA SSD 512GB
Media Drive: Samsung SSD 840 EVO 1TB
Thanks for the thorough response, I appreciate it!
A 970GTX would definitely be an upgrade I would recommend. Converting to Cineform is definitely another option you can use. The built in Cineform encoding options are just not as extensive as they use to be when Cineform had their own encoder. There are far more encoding options with DNxHR at this point which is why I recommend as a workflow option to handle that media far easier.