To explain why the sideways tunneling is harder than going straight down.
When they dug the 2nd hole they used a drilling rig truck that had all the drilling equipment and drill pipe column to go straight down.
Going sideways in this last phase is a whole different animal. The workers basically had to use special hand tools like jackhammers and maybe a hydraulic water drill which can drill through rock using a fine high speed jet of water that can cut through the rock.
It is a slow process because they dont have the luxury of the drilling rig truck that was used before to go straight down. This is why they are also using small explosions to break apart the rock and it makes their efforts easier to go sideways.
Someone earlier shared a link that indicated the hard rock that they have to go through is Quartzite which is a hard rock to get through.
Quartzite is described below:
"Hardness In its pure form, quartzite can at times be harder than granite. However, since it goes through compression with sandstone and other minerals, there is often variation in the stones hardness. Some quartzites can be harder than granite and others can be soft as marble."
https://ariastonegallery.com/what-is-quartzite/