No idea when LG will update android on the G3. I wouldn't of updated to lollipop if I knew it was going to make it slow. It'll take an hour or so to download kit kat.
Not sure downgrading will work, might have to reset it. If I get a boot loop, I'll switch to a third party ROM. Assuming I can get in the download mode.
I would probably have enough money for the Moto X next month. Kind of sucks that I haven't had the G3 very long, shouldn't need to "upgrade" already.