I can't find any helpful results when searching this issue, but for some reason, CaveStory+ is running super slow. The music is playing fine, but I noticed stuttering and some screen tearing that's making me sick. I'm forcing VSync in the Nvidia Control panel, but nothing changed. I'm running on a GTX 1660, 16 GB RAM, and an Intel Core i7. I also saw that using multiple monitors causes the issue, but I unplugged my monitor to no avail.

Having the same problem. The Steam version runs fine for me.

Has to be some sort of timing issue. I have the same issue trying to run this in Windows, sound is playing normally but the game runs at about half speed.

Running the game under Linux via Wine works perfectly.

I am also having the same problem. I've tried playing around with the few options available but nothing is helping. I'm really hoping this gets resolved soon.

Same happens to me, the first day i played it after installing it ran fine and smoothly, but suddenly the next day it started going as if it had a stuttering slo-mo effect, but the music plays fine.

I'm in the same boat now! First day everything's fine, second one slo-mo.

Okay, been testing several things, ans uninstalling it completely and reinstalling it seemed to fix the problem (For now).
I think it may be some error related to the achievement troubles (they seem to reset after relaunching the game), maybe having and not having them at the same time causes any kind of error, but i'm no tech expert so i'm just trying to blame it to any other possible error. On a side note, getting the 'toroko wins' achievement and deciding not to continue makes the game get 'visually' stuck on the introduction screen (i say visually because the menu seems to work on the background, i can hear the menu music and sounds when selecting options), maybe giving more proof that ahievements may be one of the main culprits.

Just tried the updated version 1.0.1.0: game still runs at 31~32FPS (same as 1.0.0.0).
Both the CPU and GPU are < 10% loaded while the game runs.
Tested on the same day, the Steam version runs at 60FPS and I observed no issue with other games / applications.
Re-installing the game did not solve the problem.

Windows 10 19043.1415
AMD Ryzen 7 3700X
NVIDIA RTX 3070 (472.39)
16 GB RAM

Edit 26.02.2022: solved by updating graphics driver to 511.79

I asked for direct support from gog and they replied me with a solution that helped me, so I'll just attach here the message they sent me in case it helps anyone else (but also first try to uninstall it completely and reinstalling, its faster and easier than what i'm pasting below):

For that, please try a few more suggestions:
1) Reinstall the June 2010 DirectX runtime package: http://files.gog.com/support/Directx_Jun2010_redist.exe
You will have to extract the files into some folder, then open that folder and launch the DXSETUP.exe file in order to proceed with the installation.

2) Please update your graphics device drivers:
http://www.intel.com/p/en_US/support

3) Please make sure that no third-party applications are interfering with the game. Please try to install and run the game in “clean boot mode”. Instructions on how to do that can be found here:
http://support.microsoft.com/kb/929135
Make sure the account you create for clean boot mode is granted Administrator privileges.

Edit: it seems this 1.0.1.0 update unfixed whatever i did, so i'll have to try again. However, this update fixes achievements getting re-locked and the missing holiday textures.

Edit2: repeating the two first steps again fixed the game, now it runs perfectly smooth

I had the same problem, the game was running at like 20-30fps, unplayable.
i7 6700k, GF GTX 970, 16GB RAM
The solution on my end was very simple actually - I updated my graphics card drivers to the latest version and the sluggish 30fps was gone, but after the driver update and on a 144hz monitor the game started to run twice as fast, looks like it's somehow connected to monitor refresh rate. So I had to manually limit the game to 60fps in NVidia Control Panel which turned out to be the final solution to the problem.
This version of the game is a bit sloppy and rough around the edges if I'm honest, too bad there is no PC port of the Switch version.

I had the same problem. The only solution I found was to install the doukutsu-rs mod which greatly improves the game and can be found on github https://github.com/doukutsu-rs/doukutsu-rs