I have a Komatsu PC50MR-2 with about 6000hrs on the clock. When tracking together (at full throttle) forward or reverse the machine tracks reasonably well. Whenever I use a single track the movement is weak and using both tracks in opposite directions is also weak (in either variation). The tracks are a little better when the machine is cold (first 20 mins or so). I have rebuilt the swivel joint with new seals so I think pretty safe to rule that out. The previous seals didn't look too bad to be honest. I'm now wondering what to check next and how. Am I right to suspect the control side of things given that forward or reverse has adequate power? The machine lifts itself via the blade fine, the boom is a little on the weak side and doesn't lift without using the bucket to hop it up. According to the manual, the circuit has a logic valve and a merge valve.
As I recall the blade and swing are operated by a gear pump. The main implements are run on the variable displacement pump. So you can't infer a performance issue by using the blade or swing functions. You should check all the easy stuff first. Air filter, fuel filters and any fuel strainer of water separator.
Typically I've found a lot of machines track forward or reverse relatively well since both tracks are moving the machine in the same direction. One track operation and turning both in opposite directions seems like it takes more power since there's more resistance to movement. Have you checked the pressures for the travel functions yet?