I have been dealing with this since the 24th of October and I hadn't seen ANYONE post about it on twitter or reddit. Then I discover community boards and I nearly shed a tear when I saw this post. My ticket has been tossed around like a ragdoll by the support team trying to figure out what is happening. I feel for them. Riot support please save us.
As everyone else mentioned: I cannot tab out and have to restart my pc when I get these freezes in the first 2:00 of my games
I've uninstalled/reinstalled, repaired my client, deleted config folder, changed app permissions, troubleshooted compatability, installed all possible updates for windows, removed all unnecessary background tasks, made sure xbox dvr was turned off, turned off overlays (none were on to begin with), updated my drivers, updated my firmware, deleted a bunch of storage just in-case, turned my game to lowest settings, turned off eye candy and map art, capped my fps at 120. Pretty much everything... And yes, I've tried turning it off and turning it back on again :-)
It cost me 2 placements. I shouldn't have been so bold as to try and play. 5/10/20 min leaver queues were pretty fun. I don't know how to be patient.