10 more hours on 1 hour would be amazing. 10 more hours on 5 hours would be great. 10 more hours on 10 hours would be a fine thing, but most people's working day isn't that long. 10 more hours on 20 hours wouldn't be very useful.

This is the concept of diminishing marginal returns. You cannot look at "10 hours" in isolation; its value is lower when the baseline is higher.