I tore the gun down tonight abd found a couple of things.
First, the front stock holes aren't drilled quite where they oughta be.... as a result, it was a bear getting the screws back in. There was some thread damage (dunno if I did it or the factory "rebuilders" did) that I chased out with a tap.
Second, the rear spring guide is the thinner one that seems common these days. In addition, the spring had a fair bit of grease on it. I suspect that it was the grease keeping it from twanging.
And, not surprisingly, the spring that came out of it was short. It measured just about 10" end-to-end, whereas a nice, healthy Gamo spring oughta be in the 10.7-10.9" range. So I know why the gun was slow (sorta figgered that).
I put 'er back together with a new Crosman spring. Hopefully, I'll be able to put some time on it tomorrow.