There is an excellent article about VisiCalc that goes into all the details about what happened and why, highly recommended if you are interested in that part of computing history. I was reading this section:
At its heart, VisiCalc is about numbers. One of the early decisions we made was to use decimal arithmetic so that the errors would be the same one that an accountant would see using a decimal calculator. In retrospect this was a bad decision because people turn out to not care and it made calculations much slower than they would have been in binary.
And nodding, yes, VisiCalc's successors backslid in this regard; to this day, the Internet is full of questions and answers about why Excel exhibits anomalies with numbers like 0.1 and binary floating point not rounding the way people expect. VisiCalc should really have launched an advertising campaign at the time pointing out the...
... Wait a minute. Fridge logic.
Made calculations much slower?
VisiCalc was written on the 6502, which supports BCD arithmetic. You just have to turn on decimal mode, and the CPU will add BCD bytes at exactly the same speed as it would add binary bytes.
But most numbers in a spreadsheet are simple when expressed in decimal. A number like 1234.56 takes three bytes in BCD where it would've taken eight bytes in double precision binary floating point. That not only saves memory but, if your calculation routines (which have to be done in software – the machine had no FPU) take the opportunity for early exit, also saves time. So calculation of numbers that typically occur in spreadsheets should be faster in decimal.
And small spreadsheets spend a lot of their time updating the display. Converting a number from internal representation to ASCII for display is quite a bit faster when the internal representation was decimal.
So why did he say decimal made calculations slower?