I don't think it's the cat. That problem plagued later models, and it doesn't give the impression that your engine is "as strong as before".
I think your rolling resistance sounds OK - the rear wheels have more drag from the axle & diff.
Did you check tire pressure?

Also just a thought: is it possible the brakes are engaging somewhat only when the engine is running, which would point to a booster/vacuum issue that happened after you bled the brakes? You could test by spinning the wheels with & without the engine running.

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