So I am able to reproduce the lag and lost RAM as well. I am now checking dumpsys frequently and came to the following.Thank you @Jocker111, the main problem with this issue is that we didn't find (yet) the culprit...
That "Lost RAM" increase, then decrease a little, increase again (on my side at least) and that wouldn't be a real problem if it did not take this enormous amount of RAM as you describe (until now, i never got more than 160MB "lost")
We all need to investigate in different directions to remove false positive and silly hypothesis, and then brain storm a little and see what anyone could find.
On Milestone build Lost RAM
Day 1:
444618 kb (after ~1 day)
474107 kb (20min later)
453964 kb (20min later; no app used in between)
~500000 kb (phone becomes unusable and I need to reboot)
Day 2:
-692 kb (after reboot)
148184 kb (after mild use of messaging)
346315 kb (after streaming Spotify)
Apps used:
-XDA Premium
-Spotify
-Google Now
-No graphics intense apps
I am frequently switching networks and carry my phone with me all the time. I am within the same area but switch between university wifi, friend's places and my home network. I am using the Google Now Launcher (GNL). Disabled the built-in browser.
That is all I can come up with so far. I will keep an eye on the lost RAM and try to connect it more to the Apps I am using. So far the lag (redrawing GNL, app icons aso.) starts ~400000kb lost and the phone gets unresponsive shortly after (I guess ~500000kb; cannot confirm since the terminal doesn't respond anymore). I will note down time and place for future logs.