A lot! I like to clean the gun and then look for the "shiny spots" showing where parts are touching, and I also examine and feel things that slide like the rails and the grooves in the bolt.
The rails in the receiver were awful; tooling marks were like a cheese grater. I stoned them and then slicked them with some 1000grit paper. I slicked the top of the hammer (yes an awful spot to cross when cycling the bolt with your hand). I also found a couple areas where there was a little dent that rolled a tiny piece of metal creating a lip. Those areas were certainly reducing sliding clearance and making it hang up, so I broke them off with a stone and then slicked em up with the paper too.
The bolt and carrier itself had several spots that were rough, as well as a couple more "smooshed" spots making those lips that I described before. The front of the carrier up just back from where the piston attaches was terribly pitted and had grooves and nicks in it. The carrier and bolt got treated to a slick up of the grooves where the receiver rails slide - I take and double up 1000grit around a zip tie for the shape and something to hold, and then run it through until shiny/slick. The bottom of the bolt where it rides over the hammer was grainy and tool marked badly, and the "ramps" on it were also friction points that needed a lot of sanding. You can feel the difference if you do this incrementally and feel the bolt sliding over as you work on it - as you get one side of the ramp slicked it really screams how badly the other ramp is, and you can feel where a hold-open would happen if it did. So you smooth that side out too and then she's good in both directions.
The backbone of the hammer that has the bolt slide over and push it down to reset it was very rough. The metal is pocked and it was a huge source of friction. It got slicked up and was a huge help.
The last spot that I found a problem was in the dust cover. It had a flat spot with a lip and seemed to be touching the bolt as it went past. I got that squared away too.
I haven't shot it to see if I'm done, but just hand cycling makes you all giddy when you compare how it was before you started.
I hope this helps some