"Nearly instant" is a massive loss of usability compared to "instant". The quicker the transition the less disruption there is to your short term memory. If it's instant then switching between virtual desktops is almost as good as having multiple monitors. If I have to wait for even a brief animation (and MacOS's animation is painfully slow) I'm likely to forget details, which means I'll have to switch again, and risk forgetting even more.

You can change the default slide-in/over animation to a faster fade-out/fade-in one by enabling "Apple menu -> System Settings -> Accessibility -> Display -> Reduce Motion", but you cannot disable the animation completely.