Like @RhettZufelt , I see big differences between browsers when using the new Map Viewer. The new generation of tools doesn't perform nearly as well in Firefox as it does in any Chromium-based browser, like Chrome and Edge.
I do not, however, see much difference between the new and old map viewers for any given map. The new map viewer, though, does not have the same feature limit of the Classic. So if you have > 10k features in a layer, the map may appear to be taking longer to load, when it's actually loading more features than is possible in Classic.
Back when the new viewer first came out, though, I vaguely recall certain kinds of layers not performing as well. It's been a while since we made the switch, though.
Do you have any publicly available maps that show this difference? And are you on AGOL or Portal?
And personally, I don't see any difference with Experience Builder, either.
- Josh Carlson
Kendall County GIS