Weird. I looked online and it seems good for stardew. Anyways, I looked and it seemed like the cpu is a bit subpar, so if you need it, try limiting the frame rate to 60 using the " Stardew FPS (Animation Frame Speed)" mod (cant give link due to guidelines )
Can you explain what you mean by game speed? Frame rate? Length of in-game day? Because my first thought was wanting to have the in game day last longer, but SomeoneEls has such a different interpretation that now I'm not sure which you meant?