I guess the best suggestion is to start with draining the fluid, and putting fresh stuff in.
Keep in mind it's Type F, ATF, not power steering fluild.
Maybe someone can type the procedure to manually pump the fluid out of the system to flush with new. I did it by manually turning the pump pulley, but it was kind of a cluster you know what. Lots of fluid everywhere until I figured out what I was doing. And I don't remember exactly what finally was best.
With fresh fluild, then see how it does.
A pump can struggle for many reasons, which is what the impact on engine RPM represents.
The pump could be worn out.
A line could be failing, such that as pressure builds it's getting wasted bulging the line, or something like that.
Your front end parts could be worn such that it is taking excessive effort to get everything to move.
The only way to know for sure would be with some sort of pressure gauge arrangement to verify pressures. But I don't know any of the proper values.
Just check over everything. Make sure all the tie rod ends are good, make sure the ball joints are good. Make sure all the hydraulic lines look good. Flush the fluid. Then see how it goes. It might be time for a new p/s pump. Or it could be the idle air solenoid needs to be cleaned up or replaced. The IAS should get a signal from the computer to increase airflow as the engine is dragged down by the pump. So either the pump needs more power than the IAS can account for, or the IAS has lost some of it's range.
Ouch.. more questions than answers... sorry.