Tell that to your database which is highly optimized for using integers as IDs. Using a UUID everywhere when you could be using an integer is an invitation to low-performance-city.
A UUID is only the size of two 64-bit integers anyway. And either identifier likely will be a tiny fraction of the data in a particular row. So I'd doubt this is any real performance problem in the vast majority of applications.
UUIDs look big and scary in hex notation, but underneath it's a compact and fast binary format, just a 128-bit integer.