Why does this drill have a 25 second mining time (ie. how long it takes to pick the thing up by hand)? Space Age's Big Drill has only a 0.3s mining time. There's very few situations where it makes sense to have a mining time longer than a second or two, and most of them just involve things that you really don't want to accidentally pick up by hand. AFAIK, there aren't ANY player-built entities with longer than a 1.0s mining time in the vanilla game, including things like nuclear reactors, rocket siloes, etc.
Note that this is referring specifically to the mining_time parameter in the minable field:
minable = {mining_time = 25, result = "ancient-drill"}
This parameter does not define how fast the drill extracts ore from resource patches (that is controlled by the mining_speed parameter at the top level in the entity, which you have set to 20), it defines to how long it takes to "mine" the entity back into an item by holding down right click on it.
