Some emissions equipment does not have a downside other than cluttering up the bay. Things like
The charcoal canister, Air aspirator tubes/valves, and PCVs.

Others rob power like smog pumps

Others prevent max power from being produced like EGR valves, Catalytic converters.

The above is only some of the old school stuff, on a 'modern car' (like anything post
85 ) the paraphenalia can be mindboggling and removing it difficult.