"Manual calculation" takes less time. Why?

[Kjetil]

An excel workbook (approx 10 MB) compiled from a 3rd party program used to
take 30 minutes to save.

After choosing "manual calculation before saving" saving time was reduced to
3 minutes.

Curious to know why...

Thanks in advance!

Kjetil

 

[Dave Peterson]

If you just force a recalculation (without the save), how long does that take?

(I don't have a guess when you answer it's not close to 30 minutes!)