He isn't the R&D guy. He does some basic maintenance in addition to his normal duties, but we rarely see him do much beyond sewing the costumes. Alfred has never been portrayed as responsible for development of all the bat-toys, or anything truly technologically advanced. So while Batman has a Batmobile, Alfred might change the oil or rotate a tire. He isn't going to test the explosives or debug the Batputer.
In the movies:
Batman thru Batman and Robin
You are somewhat mistaken in thinking that Alfred alone is responsible for inventing everything. Bruce won't just wave his hand and order Alfred to take care of it. Instead, classically, Alfred maintains the equipment. That said, depending on continuity, Alfred often has a background in SAS and so is about as close to a renaissance man as you get.
One of the few peeks we get is in Batman Forever, when Alfred makes a suit for Robin. Did Alfred invent ballistic fiber? No. But he knows how to sew. So while he "invented" Robin's costume, this is not entirely out of this world. Aside from the obvious question of how exactly he got all of Robin's skin measurements so exactly without him knowing.
The presumption was never that Alfred himself invented these things, so much as maintained them. But don't discount Alfred's intelligence by any means.
This is the first time in the movies we really see Wayne draw on other resources. Lucius Fox is the head of Wayne R&D, where Wayne gets much of his material. This is far more like the comics, where Batman outsources the original versions of his tech. Often Batman will find something already made, study it, then adapt it to his needs.
What we see in these movies is that Alfred is helping with testing these things and buying them- testing the face shields (which are custom versions of standard ballistic face shields) and the batarangs. He is NOT inventing or building them, he is buying them from China. Even some of the costume tricks, as we see in the first movie, are adaptions from the standard fighting equipment of the assassins.
Batman the Animated Series
There's actually an episode where we meet Batman's mechanic, who Batman also hired to design and build the Batmobile. Again, we see that Batman is willing to outsource.
Comics: way too many continuities here, but in general it holds up that Alfred maintains rather than invents the equipment.
Edit: in addition, speaking as someone who briefly in life was live-in help, it's called planning and time management. Really, that's all it is. And Bruce will have access to all of the latest gadgets to make cleaning easier. Most of the mansion is unlived in, probably locked down and left alone between charity balls, which means Alfred is mostly taking care of a fairly small suite of rooms. Bruce never seemed the type for complicated food- and judging from his body is probably mostly vegetables and lean proteins, quick and easily prepared meals.
I mean, just so we are clear here, you realize that Alfred doesn't clean the entire house everyday and cook a feast every night, right? You have a rotating calendar of responsibilities. Every day you make the bed and cook. Laundry once a week. Vacuum (including moving the furniture aside) twice a week. Dust the collection in the glass cases once a week. Daily straightening. Windows once a week.
This is not complicated. Adding in weekly tune ups on the Batmobile, ordering batarangs when you're doing the weekly ordering. These aren't a big deal.
Understand that a butler is a personal servant. He is a majordomo, but is not meant to be the sole help in a household except for very small ones. When the mansion is only occupied by Bruce and maybe a ward, his job is pretty straightforward. When balls are held, temporary help is hired- caterers, servers, etc.