Excel recalculates everything each time time you enter data in a field. If your sheet has too many or too complex calculations, it will take a lot of time everytime.
Check how long a recalculation of your sheet takes:
Hit F9 and see if there is a noticable time until it's done (in the status line, percentages are counting to 100 or such).
If there is such a delay, you can either
- switch off instant calculation (on the Formula tab, right side under Calculations). This will solve your issue, but you will need to remember to hit F9 every time you want the calculations done; otherwise, you will see stale data / outdated results
- find out what exactly takes so long, and rewrite the formulas to be faster, or remove some
- tell Excel to use all available cores (under Options/Advanced)
- get a faster computer...