I am under the impression that MS expected that W10 would sell a lot more phones, so they announced updates for almost every phone they had (even the old 520 was supposed to get it and then after months they canceled it, but not before releasing some insiders previews for it as well lol). So now that the OS is as dead as it can be, they are not worried about making it run better, specially for non flagship devices (anything other than 950)... It's like they REALLY gave up, but they just can't officially give up.
There was this interview that someone from MS said that they would still work on Windows mobile even though the sales were disappointing because that was the future (ARM processors, battery performance etc), which really translates to: we dont give a fuck about older models, we are doing this so that in the future we can release a cool Surface Book/Phone/Pro with ARM Windows, and to do that we just need to get the 950 running well because the future devices wont be less powerful than that.
I always had problems with those experimental builds - the reason why they are experimental - and never complained much because at least I could see that things were evolving. But in the last 3 months things are not getting better and what worked well now is broken.
The Edge problem is a shame, it crashes when Im surfing Neogaf mobile website WITH PICS TURNED OFF. Seriously. A long time ago (6 monhts?) it would crash only when I was reading one gigantic article from Mother Jones, but other than that it would run just fine everything else.
I have a 640 btw, far from being an awesome phone but I dont see why it CANT EVEN SAVE THE FUCKING PHOTOS I TAKE. The problems I experience seem to be much more related to the software than the hardware (I am not even turning on HDR for photos because that sure is too much for this hardware lol). But right now my phone takes more time to 'process' a normal photo than it used to take a few months ago to process an HDR photo. And yes, I did reset my phone in the meantime...
I'm having the exact same experience with my 640 too. An iPhone 6 is my main phone nowadays, but I noticed the last few builds on the 640 have been absolutely shit like you mentioned.
8.1, Internet Explorer, Lumia Camera was a godly experience compared to this Windows Mobile 10 shit.