I'm looking to purchase 2 separate flights that I will fly all in one day, morning Madrid to London Gatwick and then in the early evening, London Gatwick to Zurich. Is it worth it to purchase Speedy Boarding for both flights?
I often fly Easyjet and have never used Speedy Boarding...the flights you are taking are not that long.
No, it's not worth paying for. Your flights will be short, and there is no advantage to boarding first.
The reason for "speedy boarding" is that seats are not allocated at check-in so you sit where you can and may end up apart. But as already said it they are not long flights.
Be aware that on flights where you are taken to the plane by bus, "speedy boarders" may get on the bus first but you will be joined by other passengers who have not paid the fee. At the plane the usual free-for-all will take place. There are no refunds if this occurs.