performance issues

I have a 2004 Corolla, which I bought used with 30,000 miles on it 6 months ago. When I bought it I was actually impressed at how zippy it was, though it doesn't have a lot of horsepower. But a few months later it seemed like that zippy acceleration went away, and suddenly it was performing more sluggishly. I got an oil change, filters changed, engine flush, and fuel injection cleaning, at the recommendation of my lube place, yet still it doesn't seem to have improved the performance. Is there anything else that could be the cause, or is this all in my head?

Could be both actually. First, is it a manual or auto? Your car sounds too new to have any transmission related issues, but a tranny fluid check/change is a good first step. For an auto, brown and burnt smelling fluid means it's overdue. Fluid flushes are generally overkill, but some do it for peace of mind and the shops love to sell it.

Manual trannys are pretty simple and reliable...you should go at least 100kmi before any tranny-related maintenance is needed...except maybe a fluid change. But if you try to accelerate and notice the engine rpms rising when you're in gear and the clutch out but the speed doesn't correspond, then a worn clutch may be your problem.

You don't know what the previous owner had done, so there may have been fuel additives that can perk your car up or just the experience of being in a different car can evoke a sense of speed since you are really comparing it to your previous vehicle...its more perception than fact.

I would recommend a good fuel additive to keep the innards clean like Seafoam or Chevron Techron fuel injector cleaner. Both have good reputations in this forum.