From a guy in a group I'm in from earlier today
"Three days before departure, I booked rapid COVID tests for all of us, including kids. Test was done at 945AM Sunday at the main testing site in Reykjavik and took less than 10 minutes. I used my email as the contact for everyone and had results within a few hours. We also completed the attestation form on paper before going to the airport. I had printed them ahead of time and had them with me.
At ticket counter, gave our passports and attestations to ticket person, showed our negative COVID results on my phone, one-by-one.
No issues and everything moved very quickly for us this morning, but we'd been told the lines are long on the weekends."