Try with another charging cycle. Charge it to 100% while off and then look how it goes.
Try using apps like wakelock detector to see if there are too many wakelocks or apps like betterbatterystats to have better statistics on apps battery usage.
I still doesn't work...did you take a nandroid backup using TWRP before flashing?
If not, this is a bad habit.
If yes, try restoring it. If you were not experiencing this problem before flashing that should solve.
If you do not have a backup, try flashing the same version you were using before the latest.
As it seems that your browser consumed most of your battery, try using a different one like Firefox.
If flashing the previous version doesn't solve, try flashing a fastboot china rom, same version you are using (I mean developer/stable), than flash again twrp a xiaomi.eu.
I never had spectacular battery drains on RN2, like other people often reported after upgrading, but SOT always sucked from the first day: ~2h±20m.
So the SOT you are reporting, could be on edge, but it's not surprising me.
Recently I noticed that after it shuts down at 0% it still have some juice, once it happened that it restarted reporting 2% and it played a video for 45m before going to 0% again. And then it remained on for 30m on TWRP with screen always on.
That's why I was asking to do the same test.