I encountered simillar problem today where the default Opera GX AD blocker doesnt block ADs on youtube, to be more specific a window pops up that in the upcoming five seconds an AD will be shown, the AD isnt played but the video screen turns white with a pop up window to skip the AD, seems like the video starts playing as usual after the AD would normally end.
And also havent experienced any sort of lag.
It happens at start, end and during.
Using extension for now and it doesnt happen with it.