Both CS2D VSync and OpenGL are highly broken (in the same way).
If you run CS2D in OpenGL without VSync, there is a noticeable around 16 (+16 due to 60 FPS lock) ms delay at each frame render. This does not happen in DirectX, it is around 0 (+16 due to 60 FPS lock) ms there. Then, if you enable CS2D VSync, you can pretty much double that delay.
To be fully sure of this, I tried removing the FPS limit and then using CS2D VSync in OpenGL, even at (120 FPS locked through third-party) with CS2D VSync enabled, it still reported around 16 ms delay at each frame considering I was having 120 FPS (1/120 = 0.0083333333333333, but 1/60 = 0.0166666666666667)
This value seems to be hard-codded by the developers around 1/60 or 16 ms/15.3+.
I highly suspect this issue is due to Blitzmax, but due to CS2D itself.
The FPS unlocker I've written is currently capable of reducing both DirectX and OpenGL render delay from [DX: 16 ms -> 0 ms], [GL without VSync: 32 ms -> 0 ms], however, it doesn't affect GL with VSync: [GL with VSync: around 64 ms -> 64 ms].
Based on that, I'm guessing this should be possible to be fixed.
Is there a plan to fix this?