Using NumOfPages will slow your report down as it has to render all of the pages before it can calculate that value.
So, you have two options. You could change the formula to something like the following:
If OnLastRecord then {MyTable.MyField}
Or, you could just put the field in the page footer where you want it. Then right-click, go to Format Field, click on the formula button to the right of the Suppress checkbox (do NOT check the checkbox!) and enter the following formula:
not OnLastRecord
This will suppress the field unless you’re on the last record.