@Silwith
JavelinJoe is right. Doing magic is just another way of doing things. Especially if you do for the first time. He mentioned that some stuff was new with the slimes. And logically you have some try and error at first, which takes time.
Doing it faster is only possible if
- Detailed knowledge and experience about work and process
- Standardisation
- Preproduction of needed pieces
Even if he was doing in his previous life some construction work, only very few people have knowledge about the whole thing. Mostly you have specialized workers in a certain field.
And even if you take thousands of slimes, they would be blocking each other with limited space.