Hello DJC Clean and welcome to the Forum
Window cleaning is one of those jobs that is a lot harder than it looks. It is physical, and it will take time to develop the muscles you use. Until then, you will be slower, and most probably will ache at the end of the day.
However, £5.50 in half an hour is not too bad for a new starter, you must price your work correctly, and that is tricky.
You'll find you probably get up to twice the speed your going at in just a few weeks time and in six months time you'll be burnin rubber.
It takes time to get skilled with a squeegee, and until then you'll probably go through a fair few scrims per day. When you get the hang of it, you'll probably find that most of the time you wont need to scrim at all.
As you get to know your round, you'll remember where to place the ladder so that you can reach the most windows with the minimum of movement, but this is something you just learn by trial and error. Also, many windowcleaners find they slow down a little bit in the winter, especially if it gets icy. Come the summer and you'll be laughing.
Stick at it, and I'm pretty sure you'll find this business to be both satisfying and financially rewarding.
-Philip