I'm really enjoying the game so far, unfortunately, the game was sluttering for me especially in some particular levels like "The Toxic Sewers" or "Old Sewers".

My specs :
Win 7 64bits
Intel Core i5 2300 @ 2.80GHz
6Go RAM DDR3
1535MB NVIDIA GeForce GT 440

I found that running the opengl version of the game ("deadcells_gl.exe) and in fullscreen (not windowed fullscreen) helped a lot, and now it's almost totally smooth, except for some micro lags occasionnaly. I only have huge slowdows with the "The Watcher" boss (big eye with tentacles).

I hope this will help some, and I wanted too to point to your attention that if you encounter this kind of issue, devs are proposing a form to understand better were they come from to try to fix them. It's here !

Thanks for the tip. I switched to real fullscreen and also disabled vsync – game seems to be more fluid right now.

I'm not sure if opengl version is better for me. Hard to say, because FPS counter from GOG Galaxy overlay isn't working for me - when I tried to use it in opengl version, background disappeared and with DX version it brings just some noise to the screen, not counter. Anyone had the same issue or has solution for it?

Glad it helped someone else :)

For displaying the FPS, I'm using Fraps (you can use the free version to display the fps without issue). For me it shows FPS both in DX or OpenGL modes :) Hope this will help you too again !

It did, thanks again.

With both DX and OpenGL I get very similar framerates. OpenGL version seems less affected by occasional framerate drops. Still then, this is just my feeling and I could be wrong.

You're welcome, glad I could help again :)
For me opengl help a lot, but like I said, I still have framedrops as described in my OP. I hope they'll be able to fix this for all of us encountering this kind of issues :)

I found another suggestion (on Steam forums). Seems like Dead Cells doesn't like high framerate, for some people limiting to 60 FPS helps. This can be done with external tools like RivaTuner or NVIDIA Inspector. This doesn't apply in my case, sine I'm playing on old laptop. Another thing to try if you're looking for some improvements.

Also I seen multiple reports of Vsync being completely broken, especially when using OpenGL - doesn't limit framerate when using monitor with 144hz refresh rate and so on.