Departing from an airfield that sits at 98ft, i set an alt of 34000ft for my flight. The aircraft climbs at an incredibly slow rate and when levels out at around 13000. However, when i look at the tracker from my Virtual airline it shows i'm actually flying at around 45000ft!
What am i doing wrong?

Have you set unites as meter in P3D setting?

There is definitely something wrong, as ceiling for 737-200 is 37000 ft. How slow is "incredibly slow"? How long did it took you to get at that level? It takes me from 10 to 15 minutes to reach cruise FL370.