I left Tesco just as they were putting in the new computer system (it is very similar to optix- which leightons uses)
They have had a new invoice system which I think is a lot easier to navigate and more efficient in payment.
Asda invoicing is ok once u get used to the system which can differ according to your browser.
I generally found Tesco clinics were always understaffed Compared to Asdas.
Asda tend to put a lot more emphasis in ticking all the boxes and making sure protocol is followed- albeit in 20min!
Will vary store to store but be prepared for high pressure clinics!
Staff are usually quite good- if you’re a multitasker it will suit you.
Good luck! Maybe ask agency to put you into a new/quiet store to begin with so you get your bearings first