When we use custom validation for a cell using below method
kendo.spreadsheet.defineFunction("CUSTOM_VALIDATION", function(num){});
its triggered for each cell. Let say i have applied this validation method for 100 cells. If i changed any one cell then this method got invoked 100 times. Instead it should execute single time i.e, for that changed cell only. We cant keep 100 different methods for 100 cells. This will impact more on performance. Please look it on this and fix accordingly. 
This feature will be helpful for complex usage.