If it's that slow after lunch, that probably means it ate too much!
Kidding aside, update the app if an update is available, and try restarting your phone. If this doesn't work, try freeing up space on your phone if theres not much space available (like less than 5GB), and restart your phone.
If that still doesn't work, go to Whatsapp settings and backup your Whatsapp chats and media. Delete the app from your phone, restart your phone, download Whatsapp again and sign in. Don't forget to restore your chats and media!
Good luck