I have a KJ P226 with exactly the same issues when it was new.

It was also sluggish after a few shots. After a few shots, the pressure drops as the gun start to chill from the propane.

The culprit was the piston. The o-ring in it was pure junk. It was hard rubber and undersized to the point the gas leaked out before the slide could fully blow back. As the gun chilled, and the pressure dropped, it appear sluggish but that was from insufficient energy to blow the slide back fully.

I fixed it with a new piston. Works flawless now.

Which piston did you put into it? And what kind of performance are you getting out of it now ?

Before you start buying all kinds of parts, double check to see if indeed the slide is the not culprit as well. Does the slide bind from lock back to closed position with zero binding?

A piston change will not boost your velocity. The fps is affected by the quality of the magazine seals, the hop up seal, the valve in the nozzle, also if the spring on the magazine knocker valve is too strong it will reduce velocity. A tight bore barrel will increase velocity.

I have one and I keep mine well oiled and dont have any issues with it. It's pretty old too. Had it for about 3-4 years

I've got one, and had another before the one I've got now. No problems with gas efficiency. Sounds like you've got a problem. If it's properly lubed and cleaned, nothing hindering slide/piston cycling, I'd look at whether there's a problem with a crack causing an improper seal or something. Is there an excess of gas expelled when you fire? Wondering if maybe the exhaust valve is slow at cycling, venting excess gas.