Hey!
Sorry to hear you've started having problems with the 6.9 release. Since you mentioned you're on a Windows device, could I ask you if you're experiencing any audio-related corruption or stuttering like other players were reporting in this thread? Or are you only seeing degraded FPS? Additionally, could I ask what's normally happening in-game when you're having problems (i.e. does it only occur with certain champions or abilities? Is it happening all the time, even when walking to lane?)
Some stuff that could help would be if you could provide your system's Dxdiag so that I can get a better idea of what kind of system you're playing on, in case the problems you mentioned are more specific to your configuration. Also, just in case you haven't seen it already, we have a support page with some common fixes for performance-related problems here; it might be worthwhile to run through those to try and immediately improve things so you can get back to a more playable framerate. (Also, if you don't have a ticket open already with our Player Support team, opening one is a great way to get additional help.)
Once again, sorry that your FPS is getting in the way of your games!