Did you install RPM meter?
If the engine RPM actually goes to max and wheels are not supporting, it is the clutch issue, but strange thing is that the guys at Friends are not able to diagnose....! Did they change gearbox main seal? If the seal is leak, it also cause the clutch to slip due to oil entering the clutch assembly.
If you don't have RPM meter and you are assuming that RPM goes to max, follow the procedure:
1. If you have any doubts about the engine, undergo a compression test, if there is some problem, get it back to Friends.
2. Get the spark plugs/wires checked for any current leakage.
3. Get the silencer piping + mufflers checked for any possible blockage.
4. Get the brake equipment checked for any jamming issue.
4. Finally get the valve timing checked for any problem as there is ±10 degree option for advancing/retarding valve timing. Retard timing is also a major thing regarding sluggish engine pick.
For physical inspection, take your car to Sarfraz @G-10 I&T Centre.(0333-5101486)