Cheque In The Post.

Something that never fails to boggle my mind is that unfathomable question of why, in this day and age, do we still have to wait five working days for a cheque to clear?
What is it that takes so long?
Surely any correspondence between banks is done electronically now, allegedly at the speed of light.
Having popped into the bank last Friday to deposit a cheque, I was informed that the money will be available to spend sometime next year, at the very earliest. I could, however, speed things up by one day (wow!), by bunging the bank twenty quid. Of course, it's not actually possible to take the twenty quid from the balance of the cheque, as it hasn't been cleared yet. And to add insult to injury, the cheque was issued from another account at my bank!

Do any of you have banking tendancies, and may be able to answer this question?
I'd dearly like to know!