Honestly I have no idea how long it takes but how much harder can it be than fixing graphical issues? I'm not a dev so I wouldn't know but since Beth has not really said ANYTHING regarding the problems its what I'm forced to assume.
I have a pretty plain right to be pissed over this, the very least Beth could do is keep us informed, I know nothing about fixing bugs but I know it takes almost no time at all to have someone just tell us whats going on outside of the pretty disappointing list of bugs they are fixing.
It takes alot longer is the answer. Graphically fixes wont effect the quests so its alot easier to test the fix. Fixing a small issue with a quest will require testing to make sure a) it works, b)it doesnt break anything else in that quest and c)it doesnt break other quests.
They could have waited and put the framrate/lag fix in with a big quest patch that im sure will be out sooner rather than later, but they didnt and instead released what they have fixed now.
It will take time tho for the big quest patch because its alot more complex