Ok so this is what Hitched said in their article "32 Questions to Ask Your Wedding Photographer": Some photographers will be able to very quickly pull together a selection of unedited photos and turn it into a slideshow to show during your wedding breakfast or later in the reception. If you’d rather not see any pictures until you can get the whole gallery, then wait, but it can be a lovely touch if your photographer is able.
Honestly, from a photographers perspective, this is something we'd rather not do. Of course it's possible, however most days are already filled to the brim with activity and things that we're needing to do to keep on time and within the hours we're paid for, this would only add to the workload and might not be a perfect edit that we'd like you to see. Sometimes this is possible if there's a second photographer present who can keep shooting while you edit a few sneak peeks, however, you'd need a laptop, projector and a screen to show it on. And I used to think it was a cool idea until someone in a photography group said that it would then pull your time away from your guests as you'd feel obligated to watch it.
I think it's a very American thing to do as it seems to be popular in more American wedding photographer groups as the wedding day there is very differently structured to ones in the UK. Personally my turnaround time is very quick, usually within a week or two, for you to get your full gallery of edited photos, so I'd prefer not to be sitting on my laptop at your wedding editing photos that I've taken that morning. I usually do sneak peeks within the night of the wedding or next day, so I don't think it's necessary to be trying to do it on the wedding day. I'd rather be up and about focusing on not missing any moments!