Traditional direct debit payment takes around 20 seconds or so.
An experienced cashier is much faster in giving you your change if you pay cash.
There is a newish "instant" payment with NFC chips, that doesn't require a PIN or a signature if you pay less than 20 €. It's much faster (maybe 5 seconds?), but it's not available everywhere yet. Certainly not at shops where you typically only spend small amounts (bakery, small Asian fast-food places etc.).
When not paying contactless (which takes less than 5 seconds like you mentioned), paying by debit card takes 6 seconds: enter card, mash 4 number PIN, push OK. And after that 0 seconds spent on change, fumbling with bills and coins etc.
What I noticed in Germany though, is that many store clerks insist on taking your card, then putting it in the machine, handing it to you, then take it back from you when you’ve entered the PIN. Yes, if you do it 1920’s style then it takes longer than a cash transaction...
Yes, unfortunately. I don't know if many shops have shitty Internet connection (sometimes the card reader even displays "ISDN dialup" in the status field) and/or the online clearing takes so long, but it really is annoyingly slow most of the time.
That's a service. You'd be surprised how many people are unable to insert the card into the reader correctly.