I have a Flymo Hover Compact 350 mower which is not cutting as well as it used to: it tends to leave clumps and sometimes I have to go over the same patch a couple of times because initially it bends the grass over rather than cutting it. This is for normal lawn grass with maybe a week's growth since the last time it was cut.
I've had the blade sharpened. As far as I can tell, the motor is running at full speed.
The only thing that is unusual about the blade is that it is slightly kinked: the last few centimetres where the blade is sharpened bend upwards by a few degrees (ie the tip is slightly further away from the grass). Should the fact that the blade is not perfectly flat matter, given that the height of the cut will be determined by the lowest point of the sharpened part of the blade.