LR5 very slow performance while using the Develop Module>Adjustment Tools

Your symptoms are happening because some sort of unusual problem has occurred (or is occurring). Although I agree it is probably (not necessarily) due to some insufficient robustness / bug(s) in Lr5, you can probably solve this problem without Adobe's help. It's usually a system problem or a critical Lr data file problem...

To be clear: 99.X % of users are not having this problem, so asking Adobe to "fix this" may not happen without some more specific info from you, or maybe you'll get lucky and they'll stumble into something... Obviously the app was not performing thusly for the developers / QA.

The other possibility is that it has absolutely nothing to do with Lightroom, and so it will never (can't ever) be fixed. e.g. apps that don't use that 31st bit of the 119,001st byte of the 4th memory chip aren't affected if that bit is stuck, even if that app is Lr4. Although I chose a hardware problem as example, ditto for software/data-file problems.

Bottom line - the fact that Lr5 isn't running well, and Lr4 was, in and of itself, doesn't mean much, except, "you got a problem" (or "Lr is having a problem", if you prefer)... - consider solving your problem if you can, and then tell Adobe what you did, so they can use that info to robusten for the next user(s).

Something specific to try first: Delete Lr preferences (.agprefs file) and/or try with a virginal catalog. If that helps, and you can recreate the problem by putting the old files back, then send the offending prefs file and/or catalog to Adobe for evaluation.