Shimano 105 v Tiagra I have a new Giant Fastroad, with 10 speed Tiagra, my old bike (7-8 years old) has 9 speed 105.
I think that the 105 was smoother even on the old bike, would the 105 be compatible to fit to the new bike and be able to cope with the extra gear?
Shimano 9 speed has a different pull ratio to 10 speed.
So this wouldn't be possible.
If your Tiagra is the newer 4700 series - the cabling is routed under the tape - which gives a cleaner line but I found on my Shimano 5700 105 (vs the older 5600 external routing) gave a stiffer shift.

I have found on servicing my cables with lubed and sealed Jagwire - the shifting has improved greatly though.