RD adjustment question.
 

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.

How old is your cable housing? That short section of housing that's adjacent to the rear derailleur can be particularly troublesome.

+1
Sounds very like the problem.

everything in the drivetrain is either new or as good as new. All cables and housing has been replaced with teflon lined SS stuff...no kinks, nice big loops so no kinking and the line into the cable stops is straight too.

So far I've tried a few different chains, cassettes and last housing loop to RD. Shifters are as new and I've cleaned them out so ther's no drag in them at all. RD has had a solvent bath and new lube applied.

I have put together my share of bikes and this is the first time I've had this happen. I'm out of ideas.

Whenever I have a mystery like this, the path to the solution is to just start going through each subsystem under scrutiny til you find an issue, especially when all the parts should work together, but don't.

As the other posters indicated, slow downshifting indicates either cable friction or weak springs. Try pulling all the cable housing loose from the stops, even though it's new, and check for internal burrs, SIS wires intruding upon the inner wire, grit...any source of friction. Sliding the housing over the inner wire by hand should tell you if there's an issue.

You may also check chain length. A chain that's too long won't give as much spring tension. You should be able to shift into the big+big just barely without locking the chain and the small+small with tension still on the chain.

About the only other factor I can think of may be using a derailleur meant for wide range on a cogset with narrow range. Although Shimano derailleurs are generally more forgiving than SRAM derailleurs, they are designed for a certain range of gears, and the corresponding angled path that the RD takes differs.