Please check that your Esc is giving full power. Check by connecting motor direct to battery. Possibly you
could increase battery voltage, subject of course to motor and esc limits . John.
Remember the electronize range of esc have a 'speed' screw top right i think you should adjust this when the esc is receiving the transmitters signal for full throttle, there is also a 'L-V-H' screw hole which i expect is set to V at present which is 'variable' you may wish to turn this up to H ( high) and see if anything improves with the adjustment of those two settings.
Be sure to follow the instructions that came with the esc as i typed this out by memory of setting up a club member's recently and be careful with the screw adjustments use a watch makers flat head screw driver ( 2mm head i think).
If after all that has failed to do anything worth while, then look at changing the prop to either a larger pitch or smaller prop 35- 38mm as a good starting point, buy plastic ones to start with once you have a good setting then purchase the prop in brass its cheaper in the long run for 'experimenting'.
If none of those do anything.... try upgrading to a 600 size motor as they can still be used with your esc which i'm assuming is a 15amp? if its the 30 or 40amp esc you can go much higher in motor size 800-850's but this may be to much power!