Yes processing and response time is important. Some points/scenarios to keep in mind when writing ATM testcases are...
1. The denominations of the cash dispensed by ATM. The 100s, 500s or 1000s notes dispensed by ATM would depend on the amount requested by user. Like for smaller amounts 100s are enough, for medium amounts 500s and 100s and for larger amount we can have a combination of 1000s, 500s and 100s
2. The receipt generated should correctly reflect the amount withdrawn and the balance
3. If ATM machine provides other services like changing pin number, updating customer bank records, paying bills etc
4. The instructions and graphics should be clear. we should use arrows for service selection if buttons are present OR if it is touch screen then it must be clearly mentioned where customer can tap on the screen
5. The card pin and account validation