When you have a part with many features and want to edit one or more dimensions in a sketch , the new preview function makes this extremly slow. Particularly if the sketch is in a feature near the top of the model tree.
Change one dimension and you have to wait quite a long time for the preview. The more dimensions you want to change, the more time.
And you also have to wait for the reganeration when you are finished.
Can I turn this feature off?
There is one config.pro option that might help you. It's called dynamic_preview and has three values: attached (default one, it's the new preview mode that actually creates geometry during preview), unattached (old-style, it shows only outline of feature being created) and no (as you can guess, it disables preview completely). So, according to your needs, you could set dynamic_preview to no and disable preview. Downside is, you'll do it globally for all the models and features, so partial solution could be to change the option while working with large models and put it back on for smaller ones.
Bumping an ancient thread, but wanted to link a different answer which was the one I was looking for (assuming you're editing dimension values in model mode, not in Sketcher):
"enable_auto_regen no" is the option that worked for me.