There should really be no such thing as slow to the person playing the drums. Whatever tempo the listener is hearing, the player is going to multiply it as necessary to make a comfortable, easily-maintainable pulse out of it— subdividing, we call that. Making an easy tempo out of a hard tempo. So if a tune is counted off at 40 beats per minute, you would be thinking and feeling 8th notes or 16th notes (pulses at 80 or 160 bpm, respectively), while playing a feel that states that slow 40 bpm tempo. In the practice room, playing to a metronome setting of 40, you would be feeling a pulse of 80 or 160, while you could be playing an actual tempo of 40, 80, 160, or 320.
Approaching tempo in this way, just the bottom 16 increments on a conventional metronome (40, 42, 44, and so on, up to 76
) will cover all the tempos you reasonably need in actually playing music. Using every other increment is nearly a fine enough gradation for the practice room, since:
40 = 80 = 160
44 = 88 = 176
48 = 96 = 192
52 = 104 = 208
56 = 112 = 224
60 = 120 = 240
66 = 132 = 264
72 = 144 = 288
With just those eight actual settings you are also covering: 80, 88, 96, 104, 112, 120, 132, 144, 160, 176, 192, 208, 224, and so on— above that point, I generally start counting tempos in half notes. The wider numerical gaps at the top of the scale are not important; the real, felt difference between 192 and 208 is the same as that of 96 and 104. If you ever wake up in a panic over always playing 120 and 132, but never 126, you can change things up and practice every other increment starting with 42, for awhile. See if it makes any difference. I don’t believe you should be adhering systematically to one set of numbers, anyway; the point is that by subdividing, a fairly small range of tempos will cover the entire spectrum.
In my own practice, I generally try to use the slowest metronome settings possible, often below 40 bpm. This does force you to subdivide accurately, and once you can do that— it’s not an advanced skill— it really isn’t notably more difficult than using a faster click.