I got this game last night on sale, but every single time I play I get serious lag/stuttering (especially when it rains). I've tried various things, but nothing seems to help. I can get onto the game, but the lag/stuttering always happens. My computer is windows 8.1 64 bit.

Do you have a graphics card? If you do check if the game runs with the integrated graphics card. Seems weird because this game is easy to run. Also are you using any mods?

I think my graphics card is an AMD radeon hd 7600g card. The game was really laggy before I got Smapi and cjb cheats menu mod. The game is still laggy now especially when it rains and becomes unplayable. I even tried the task manager fix thing and it didn't help.

My computer can run Genshin impact (sometimes with a bit to lag), so I don't know why it can't run this game without the problems.

I just purchased this game and dumped a ton of mods and even though the computer I'm using is 5 years old the game seems super sweet.

it could be that this game is not really compatible with that graphics card.. A lot of games have trouble running on integrated graphics card (I assume you use a laptop) . Maybe try to lower resolution, disable some settings and see what happens. Also if you install mods don't install "PyTK - Platonymous Toolkit" because If I remember correctly the mod causes a lot of stuttering.
EDIT: maybe you could also update your graphics drivers If you haven't already. (you can find them in the AMD website)

I've updated what I can and it hasn't helped. I do use a laptop to run this game (I have no other computers, and I doubt my mother's Chromebook could run it either). The only mods I have are Smapi and cjb cheats menu, but the lag happened before they were installed. The smapi mod comes up when I'm booting the modded game up, but I get a bunch of red writing saying something about not being able to install the mods into my game but the cjb cheat thing works. I didn't go to any site to update anything, I just clicked on the graphics card thing and tried to do it that way and it said they were updated.