A number is currently stored as float. Could we possibly support an integer format as "calcext:value-type" attribute ? This would allow speed and memory optimizations for files created with Calc when the file is opened and when it calculates with integer internal representation instead of float internal representation. However the Open Document Format compatibility would be supported by the "office:value-type" attribute. If a number belongs to the integer set then it belongs to the float set. Therefore the reading of the file with an other tool would keep the data but it would loose the integer typing (it would only decrease the performance).
What makes you think that an additional integer value-type would speed anything up? Internal storage and calculations are in double anyway, having to differentiate between integer and float during save would just slow down things, and performance gain during load would just be marginal, if any at all. Unless you want to majorly rewrite the engine there's no advantage in integers.