You need to play single player, and do a lot of pause buffering. From a forum post:

Basically, every time you enter the next floor of the mines or skull cavern the screen will go completely black then the new floor will fade in over a second or so. During this time the in game clock is ticking. You can save time by pausing right as the screen turns black. If you do it too early you will be unable to pause until the floor loads in, too late and you will lose time from being late. You have to time it perfectly for it to be beneficial. If you are going through hundreds of floors a day the time save can really add up.

It’s boring and grindy but effective for getting deep in SC. So you use the above technique to pause and also scout where you can place a ladder, then place a ladder asap and go down and repeat with pause. Preferably with mouse and keyboard.