Why are my Shimano R540 SPD SL Road Pedals incredibly stiff to clip out of?
If you have the tension adjusted all the way down they should clip out easily. Two things to check:


Is the cleat tight on the shoe? If it can rotate at all, you won't be able to clip out.
Are you rotating your foot outwards to unclip? When I first got my clipless I tried to rotate in, and that doesn't work too well.