Build 10512 has some serious issue(s) with the Lockscreen (delay in painting the screen) and Start screen (significant number of testers reporting "Loading..." and "Resuming...").
I equate these major class of errors (we can consider them "errors"/bugs as they definitely are not the expected outcome of the test scenario) to a critical bug that needs to be addressed with a critical fix (not a 3 weeks later fix).
You think delays, being slow, and constant "resuming" and "loading" represent critical errors. Nobody in the software industry would agree with you, but you're entitled to your opinion.
A critical error/bug is a literal show-stopper. It's a feature that should work but doesn't, and ends up making testing impossible or meaningless. This is the definition typically used by anybody in the industry, including those who finance custom software development but aren't developers themselves. Having to wait longer than you'd like for an app to resume doesn't qualify.
Granted, substantial delays can make testing a pain in the rear, but if you CAN test it, then there is no insurmountable (critical) error.
Lastly, what you think should be fixed in a matter of days and prompt MS to release a new build ASAP, may very well involve a team of people working for one or two months on SoC optimisations. What you want may simply not be possible. At least the loading/resuming issue is likely systemic, for which I doubt there is a quick/easy fix.
Anyway, the point is this: you can rant all you want, but you won't get anybody at MS (or Apple, or Google, or anywhere else for that matter) to change their mind on this. The only viable alternative would be to NOT deliver a preview at all. If a majority of people adopt your view, canning the insider program is MS' only reasonable alternative, because your expectations are not realistic for software in this phase of its development.
If a bug prevents a significant number of testers (Windows Insiders in this case) from accessing the system, testing core functionalities and new features/improvements (such as the Lockscreen and Start screen bugs do), then you immediately address the bug and release a next build, so that testing can resume in earnest.
This we agree on. Apparently we disagree only on the definition of the word "prevent". A computational delay can't prevent you from testing anything. The critical error here is actually your impatience, which is easily remedied by going back to WP8.1.
If the Windows Insider Preview Program is truly for the purpose of testing by interested users prior to release and incorporating feedback into the product, then you would expect that MS would want to get the most out of the process.
Actually, finding and reporting bugs isn't the primary purpose of the insider preview. If that's what you're doing you're wasting your time. MS has their own professional testing staff that delivers far more useful, reproducible and actionable bug reports than the community ever could. What you should be reporting on are things you do or don't like. That is far more interesting to MS.