The most likely reason your video files aren't playing smoothly is because they are H.264 / AVC or H.265 / HEVC encoded files. DaVinci Resolve may be relying on your system CPU to decode these complex video files before handing over uncompressed image data to your GPU. Read more about this in my article XAVC / XAVC-S / H.264 / HEVC and DaVinci Resolve. It's likely that your CPU is the bottleneck if these video files are not playing back smoothly. The solution is to decrease the timeline resolution to HD if you are working with 4K video files, and to create optimized media or use render cache on your timeline.