Well, lets get some tech stuff answered first.
Your media is long-GOP H.264, made in-camera by specialized encoding chips. It's the nastiest stuff to edit in an NLE created by man. There's a complete frame that's "only" heavily compressed every 9-30 (if you're lucky) or even out to 120 frames with some drones & other devices, called an i-frame. In-between the chip makes data-sets of pixels that have changed since the last i-frame, will change before the next i-frame, or both. There are now encoding chips that even create partial i-frames, which is how they string it to 120 frames between full i-frames.
To play that back, the CPU cores 'link' with the RAM, and decompress an i-frame, store to RAM, call up the next frame's data-set, recall the previous frame from RAM, compute that 'new' frame, store to RAM, rinse & repeat.
For a computer to play that back at speed in an NLE with the heavy CPU load the NLE has internally, grabbing bits & pieces of video frames from many different clips, requires a hunker of a rig ... 8-10 cores running at least above 3.8Ghz CPU speed, with 10-12GB of RAM per core.
I know a number of colorists, who all run truly monster machines. When the shotlist for any job they take includes long-GOP media, that is automatically transcoded to a "proxy" media,and the transcodes replace the original clips on the sequence they're grading. (That's what Resolve's "optimized media" is ... transcoded proxies.)
So ... with the media and what you need to really play that back well established, look at your specs ... 6 very slow cores with only 2.67 GB of RAM per core.
You need to use the proxy media setup that PrPro has. On the Media browser panel, click the wrench icon to get to the Settings dialog, and choose "Create proxy media" button, and then the Cineform preset. Don't worry about matching frame-size, only aspect ratio of the media. Then click ok, and back on the Media Browser, check the "Ingest" checkbox.
Now, importing new media will bring up the MediaEncoder app to create the proxies and put them in the PrPro project bins.
For already imported media, select the media in a bin, right-click "Create proxies" ... and again it will use ME to make them. It can take a while, of course. So do this when you're taking a break or going for lunch or dinner.
On the program monitor panel, click the + icon on the lower right, in the pop-up for additional controls, hover until you see one that is the "Toggle Proxies" icon, drag that to your program monitor control block. When clicked so it's blue, you're viewing proxies. Clicked off (gray), you're viewing original media. That should help a ton.
Also, you can go into the Program monitor's playback options drop-down just to the lower right of the screen in the program monitor, and set that from Full to maybe 1/2 or 1/4 playback resolution. So playing back, it goes to a lower-res mode, but on pause, goes back to full image res.
Did you follow Neil's advice, especially regarding using proxies? Let us know how that worked for you.