An (analog) slide rule is a whole lot faster than doing digital arithmetic. But it’s approximate, yielding about 3 digits of precision, or maybe 4 digits on a large slide rule.
With an abacus or with written numbers, you can get 10 digits of precision (or 50) if you are willing to put the work in.
You can iterate slide rule calculations to get more precision, too... You're not restricted to the width of your abacus nor the number of sigfigs you can get in a single slide rule calc..
With an abacus or with written numbers, you can get 10 digits of precision (or 50) if you are willing to put the work in.