It is worse after it first comes out of standby - if you go back to TV and then try the guide again, it isn't as slow (but still slower than normal). It happens any time of day though.
One other thing I've just noticed after a week away is that the box is regularly missing recordings - if I look in the recording hiccups section, it says that the reason is because the power was lost, but the box is always left on and nothing else shows signs of a power cut so I've no idea why that would be happening?! It seems to be in the evenings around 7-9pm that this is happening.