The CPU emulator tries to be pretty accurate about timing, but the I/O is not throttled, so that might be unrealistically fast (depending on your Internet connection).
You can reduce the mouse speed a bit using Preferences.app.
The fundamental issue is that there’s two acceleration curves being applied (the guest OS’s and NeXTStep’s). I experimented with the unadjustedMovement option (https://developer.mozilla.org/en-US/docs/Web/API/Element/req...) but it didn’t seem to help (and if anything, made things feel even harder to control)
Author of the post/creator of the site here. To save everyone the click, NeXTStep 3.3 is probably the best release to try out: https://infinitemac.org/1994/NeXTSTEP%203.3 (it was the last release that ran well on the original NeXT hardware, OPENSTEP 4.0 and later were mostly focused on Intel support).
If you'd like to play the NeXT version of DOOM and explore a few other apps, they're in the "Infinite HD" drive (you can get to different drives by clicking on the computer icon in the Workspace file viewer).