Dear friend, if you have done everything, and all are good, but still starter go slow, then the final solution might be this.
You might have over-advanced the ignition, which can cause by a tensioned timing belt. all these are controlled by the CPS, crankshaft position sensor.
It's located above the thermostat, it is a round thing, with 2 bolts securing.
Before playing with it, mark it's position with a marker.
Just release 2 turn both bolts, and while starting car, move it a bit (1-5 mm) left or right.
you will notice the motor to turn more freely or more slow during starter.
In my opinion, japanese-korean starter hardly can fail.
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