I find the UK Vodafone website to be very slow too.
The speed is fine if you visit the site just normally to browse, but once you log in and start accessing your personal data it just grinds to a halt.
I've given checking my online bills. I just trust it to be correct.
As a solution perhaps, can't you top up over the phone? I used to some time ago with O2 when I was using a Pay As You Go SIM card. It was an automated service where I just keyed in my card details.