[eyerony]

The fluidity is about keeping activity on the screen tightly in sync with motion on the trackpad and having zero or no unexpected behavior or jank when e.g. reversing a motion partway through to undo it (say, starting to slide three fingers up to expose all your windows, stopping to peak at something, then sliding them back down to put everything back where it was—you can even stop in the middle of it to make the windows do a little dance and nothing goes badly wrong or far out of sync). It nails that stuff even on my aging low-specs-even-at-the-time 2013 MBP. It's not so much about raw UI speed which, yes, light Linux/BSD window managers still hold the crown for (outside dead or niche operating systems like BeOS and QNX or maybe even old versions of Windows, which actually hold the crown)

[waddlesplash]

BeOS lives on in Haiku (https://www.haiku-os.org/) which is both still alive, and blows most "light Linux window managers" out of the water in UI speed. :)

[29athrowaway]

I have a VM with Haiku and I can confirm it is indeed very fast.

I am no Haiku expert but the Haiku's window manager does not support window composition and does not use GPU acceleration, right? It is also not modular and cannot be replaced.

You can disable composition on XFWM and make it run a little faster on lower spec hardware. You can also use a WM like i3, Windowmaker or Enlightment.

IMO, XFWM is the right balance between functionality and performance.

[waddlesplash]

The whole point of Haiku is a holistic approach to system design, so no, you cannot easily replace the window manager. You can, however, write "decorator" plugins which draw different window borders, write shortcut plugins for "tiling", etc.

It indeed does not use GPU acceleration; Haiku as a whole does not have 3D drivers either (but there are plans.)