The walk on arrival depends on your airline and where you're flying from.

Domestic arrivals have a short walk - obviously, not relevant here, as you're asking about immigration.

Ryanair arrivals have a longish walk - up to 15 minutes or so, with ups and downs.

Other airlines' arrivals use the people mover, so it's a bit quicker (although being off first is no guarantee to getting an advantage at immigration).

As for timing, well... Really depends on time of day and time of year. Peak time in the holiday season? Pack some reading material. Mid-evening during term time? You could be out in 2 minutes. End of August will be very busy. Unfortunately for you, 0810 is a major arrivals bank - there are 20 international flights landing between 0740 and 0840.