They scan everyone's passports, they don't just waive you through, so I suspect it will take 1-2 seconds longer to see that that particular passport has an ETIAS associated with it and hand back the passport and away you go
1 or 2 sec's is a bit optimistic! don't passports also need to be stamped? so say 20 seconds. so every 180 people is an extra 60 mins.
So, halve that to 10sec's, a plane full of tourists will take around an hour extra.... how many planes land at Palma per day?
When new restrictions were put in place throughout europe following the Paris attacks, delays at places like Palma were in the hours.
So unless the europeans (not just Portugal) put in more booths then expect significant delays, but brexit is about going back in time, so maybe we'll travel less? drop the numbers down to a few million wealthy tourists p.a. (as per 1970s) from the 70million who currently travel from the UK.