A lack of "smoothness" implies there are hiccups somewhere, which is caused by the game stressing one component for a short period of time. However, it's hard to tell what's causing these hiccups with the information given.
For example, here's the possibilities of what your problem could be
- If there's a lot of things going on when there are hiccups, the CPU is probably being too taxed and increases the frame time.
- If you're playing an open world game or a game that streams assets in, the storage drive isn't fast enough to supply the game with the data it needs in a timely manner.
- If RAM usage is rather high, the OS may start swapping things in and out to the page file.
- If the PSU or motherboard/video card VRM isn't up...