I tried to put a ticket in on this issue but the ticket system appears to works only for "live" systems (yawn!). I'm developing on MAMP (offline). So, I'm trying this route.
Anyway. I've just upgraded to J4.2 today. It might be coincidence but there appears to be a delay between form rendering and conditional logic processing. The delay was probably always there of course, but now I see everything (every block on the form) for about a second or more before the conditional logic kicks. I am using SHOW only in the logic. This might also be the issue of course... the system has to check every step before hiding elements. But the logic really should be quick, and I'm sure I didn't see any noticeable delay before J4.2.
Anyone else seen, seeing, or had this issue?
I also remember seeing on the forum (some time ago) that someone had a problem with multiple pages displaying before the logic kicked in. I can't find that post now, unfortunately.
Cheers.

(update) I've now checked all possible combination - hide/show/different forms, etc. It happens with everything. All blocks display before logic then hides blocks as needed (1 to 2 seconds later). So, I guess this could be an issue created by the introduction of Joomla 4.2. RSJ update soon perhaps? If not I'll have to sort in Javacript.

Just in the process of upgrading some sites to Joomla 4 and I can't say this is a proble I've experienced. What has caused me a major headache is using PHP 8.0 and up, lots of strange things happpening which don't happen on 7.4. I just wondered which PHP version you are using?

Thanks for your input. I'm still on 7.4.12. I've tried everything to resolve this issue (introducing delays, etc) without success and it appears to have only raised its head with Joomla 4.2. So, for the time being I've had to revert to dropdown top menu selection of a number of different forms rather than selection withing a single form. Not the greatest solution but such is life.

A little more on this subject... I now find that "styled" checkboxes (with advanced form fields) doesn't perform well either. Checkboxes are displayed as "standard" checkboxes and standard label formatting for about a second or so before the styled checkbox and any label re-formatting replaces it. Its a VERY ugly look. It looks almost like an animation gone wrong. But... introducing a fadeIn with keyframes at least makes it visually palatable. And this is with Responsive, BS2, 3, 4 and 5, or uikit, etc... it doesn't seem to matter. It may well be how the overrides are managed (and perhaps that's changed in Joomla4.2). Something maybe RSJ needs to check.

not sure If this will help anyone else but I just solved by adding in som javascript straight after the form

// get elements with classes "rsform-block-dim-small", "rsform-block-dim-medium", and "rsform-block-dim-large"
const elements = document.querySelectorAll('.rsform-block-dim-small, .rsform-block-dim-medium, .rsform-block-dim-large');

// loop through each element and apply the style "display:none"
for (let i = 0; i < elements.length; i++) {
elements.style.display = "none";
}

That's a really great piece of work, Derek. Works a treat. Up until this point I've been using Animation and Keyframes to cut out those milliseconds of "pre-display", but your solution is so much better. Thank-you!

After trying Derek Webster's solution on a number of forms I've found that this doesn't work for me where I have quite a long list of hidden conditional elements. A zero delay / short fade-in is still my best option here. It doesn't look instantaneous but the 200ms or so fade-in looks okay and removes any "pre-display" where I need lots of those form elements to be initially hidden.