Why is so much smoke suddenly escaping from the lid while cutting?

[kittski]

I agree with @Jules and this sounds like a classic example of the fan needing cleaning. (Especially the part about the issue getting slowly worse.) When was the last time you cleaned the rear exhaust fan?

These may help

Cleaning Your Performance Series Glowforge

Caution: Dirty Pictures....or...How I Cleaned the Exhaust Fan

Update: 12/14/2018: Happily, the Glowforge crew has released an “easier to implement” cleaning regimen for the exhaust fan now…instructions for how to do it involve purchase of a special non-flammable degreasing spray cleanser and use of a special cleaning mode of the software…instructions here: Written up below is the old way that I was doing it…it’s a fairly labor intensive alternative that has worked very well if you are willing to keep up with it as you go along and not let it get …