If I notice my Civic's performance slipping rather quickly -- she's not as fast as she used to be, and that's that-- what should I do? It's not a REAL problem, so I worry that the dealership might just turn me away. What could be the source of this?

Have you had a tune-up? Maybe you just need new sparkplugs or something simple a tune-up would fix.

I doubt the dealer would turn you away. Loss of performance is a genuine problem to be solved - but be prepared to describe it. Is it a top speed issue, or acceleration/pickup?

do a tune up. spark plugs, wires, ignition coil, cap and rotor, change oil, trans fluid,!! If that dont work, check yous compression. Yous might wanna use lucas oil to. Its thick so it may raise yous compression a lil

I agree with the tune up suggestions. I've also had some issues in the past with a dirty fuel filter or problem with the fuel line.

In that case, there were specific times when the performance would noticeably drop off.

I agree with you. Tune-up should really help. Check out with the dealer, it should really help as it is not a very small issue. Which model is your civic?