I updated my smart ultra to 6.0, core os took 3min, but 'optimising' apps took ~20min? for 45 apps. Google photos had a double up problem that came right after another reboot, but the home screen started crashing (!). Fixed by installing google now launcher after a reboot to get that going again.
Battery life seems worse, even though there's a new "doze" mode implemented for most apps (switcheable). Phone no longer lasts through the day. I did a fast boot mode cache wipe, but apps like chrome browser still seem to crash often (2-5 times a minute sometimes). I tried firefox, but no better than chrome.
I've turned off google-now screen search, but still no good.
No application updates are pending.
1/ Is this just my phone going bad now just a coincidence, or are other people having trouble with Marshmallow?
2/ Is there somewhere to download the ROM for lollipop to downgrade? Will the 'optimised' apps then work?