Zoran,

Look into the clutch if its a manual.  It may be slipping at high RPMs in which case your mileage would suffer.  You can check by putting it into 3rd or 4th gear and let the clutch out.  It should stall quickly.

Has it always been sluggish, or did it begin that behavior recently?  If its a clutch it wouldn't happen overnight.

Try swapping a known good AFM into it if the clutch is OK.

HTH,

John Haddad