Question adobe premiere and preview playback at half resolution



i recently upgraded a old build for a friend, that is used for video editing mostly, with adobe suite, premiere mostly

currently it has a r7 1700x not overclocked(motherboard is cheap and heatsink is not ideal), 16gbs of ddr4 2667mhz in dual channel, a ssd as boot device, a couple hard disks and a gtx 750ti

at the moment the only old parts it has is a old hard disk and that 750ti because in recent months we changed psu, case, heatsink, added ssd and hard disk, well, alot of things making it a new build basically

the r7 is a massive upgrade from the old cpu, but when watching the video on the preview window, well it has to be watched at half resolution because it stutters alot at full resolution

being a project at 1080p we got abit confused, supposedly the gpu should help on that preview but we see activity of around 1 to 4% from the gpu reported by the task manager, perhaps only 10% from the cpu at best

to export the project afik, all is done by the cpu, this cpu is a monster, it doesn't even use all 8 cores, it uses like 6 and still does the export in 2 or 3 minutes, so we are happy with that part, but the preview is really a problematic experience

perhaps if anyone knows what we need to modify to make it work better, or perhaps change the gpu itself, any help would be appreciated