Hey, sorry to hear that... Not looking for any excuse but since the game was made in under 14 days, I have to admit I skipped the optimization part a bit :D The game relies heavily on the GPU and with a good CPU only, I think it'll still lag at some point...
I've uploaded a new version (1.0.5, you can check it by pressing Esc and opening the pause menu) with a slightly better optimization for Mid-end and Low-end quality (having to do with water reflection mostly) and I've added a performance setting which is the lowest I can go (no water reflection, no decals, almost no grass, no background meshes, low fog quality). Tell me if it helps!
Cheers!