I built up a MTB as a commuter. Deore XT 9sp RD and shifters, Tiagra 9spd chain and 12-25 cassette.
The shifting is very sluggish and sometimes the only way to get it to drop a gear is to shift it twice then pull up 1 to where I want it. The RD moves the requisite distance so I am ruling out cable/shifter drag. B-screw set so the top idler pulley is as close to the sprockets as possible and the hanger isn't bent. Once in gear, there is no chatter at all. L and H stop screws are set accurately. All the components are in new / excellent condition.
What have I missed? I don't think mountain and road cassettes have different spacers.