How could entering a 4 digit PIN be slower than "printing a piece of receipt, handing over a pen and waiting for the customer to sign"?

When still requested, the signature is often just scrawling something into a signature box on a touch screen of the same device with the chip reader or contactless receiver. There might be some stylus dangling on a tether and often a spastic finger tip is sufficient to satisfy the UI.

Generally (at least in my part of the US) people stopped being asked to sign for small purchases several years ago; usually now convenience stores and big box stores under a certain dollar amount just ask you to tap a button instead of sign.

This is the same with PIN in Europe (at least for contactless): Below a certain amount you do not have to enter the PIN.

Signing is very rare these days, non-existent for small purchases... and still seemingly rare for larger ones. I've spent hundreds of dollars in single transactions without signature or pin.