Thank you for your order. It will take some time to finish cooking. Thank you for your patience.
Depending on the difference in language ability, a detailed explanation could turn into a nightmare. Most people will accept a simple 'it will take some time to finish cooking' as an explanation. Or even 'it will take time to complete your order'. A simple answer that is preceded and followed by a 'thank you' will make things easier. Politeness is key. Adding 'for your patience' will preemptively prevent too many questions while reassuring them that you are doing your best to hurry. This is a stock answer for many individuals in the service field in America.