The right track on Hitachi EX270 does not seem to pull as strong as left. Pulling straight, no difference is noted. On turns the left track will pull harder than right. Backing up is the same. Otherwise the hydraulics seem the same. Could it be as simple as a pressure relief losing its calabration? Or worse?
More likely you're losing one of the seals in the rotary joint where the oil passes from top to bottom and it's bypassing internally. Or the motor could be getting weak. Start with checking pump pressures, then probably motor case drain flow.
You didn't mention the age of the machine or the indicated hours. I suppose you have checked that the pedals are moving through their full stroke. The mat traps a lot of dirt and can limit one side or the other. It has been my experience with Hitachi that when a swivel seal goes out the weak track will switch sides when going in the reverse direction. A quick test I have done was to walk the machine up a steep hill with the idlers up the hill checking for the slow track. Then turn the carbody around and go sprockets first up the hill and see if the slow track switches sides. If the same track is still slow you need to follow lantraxco's advice and install some gauges and do some more troubleshooting. It is not uncommon to have one pump run slow and need to tweak a screw to make it last a little longer.
On that model Hitachi the front pump works the right track and the rear pump works the left until you use both the dig function and travel at the same time then one pump works the dig and the other the tracks through the flow combiner valve.If it is weak in both directions it would indicate a weak motor and the easiest way to tell is to pin the track and check the case drain like lantraxo said. Also check cycle times.The rotary manifold can be checked by pinning one track and lift the other off the ground and apply pressure to the pinned track.If the lifted track tries to move then it would be the rotary manifold.You can check a weak pump by checking pulldowns from high idle with a tachometer.Pin both tracks and stall one at a time.They should be within 5 rpm of each other.