I can't speak for long haul travel but I can detail my recent experience of flying from London to Malaga.
Heathrow was relatively busy at 07:00. I was flying business class so straight through to bag drop however the standard bag drop desks weren't overly busy. Security was via fasttrack and this was only a couple of minutes. Terminal 5 was less busy than normal but not drastically so. I had use of the BA lounge and this was where I first noticed a big difference. Social distancing was in place amongst the seating and there was no self service for food and drinks, instead you ordered via an app using a QR code near your seat. It was a limited menu and once ordered it was delivered fairly swiftly. Masks were required except when eating and drinking.
As I was flying to Spain there was no requirement for proof of vaccination or a negative Covid test although I have proof of vaccination from the NHS app. Nothing different about boarding the flight. Masks were required to be worn at all times except for when eating and drinking. The flight was just under 3 hours which is the longest I've worn a mask and the ear loops started to cause pain behind the ears. I also couldn't read as my glasses kept steaming up. There are masks that do not loop behind your ears so I may consider those in future.
On arrival at Malaga I walked straight to passport control, two agents working and my passport was given a brief glimpse, stamped and I was waved through, all in less than a minute. I was then met by staff who needed to scan the QR code on the Spanish health app that is required to be completed prior to entry into Spain which is essentially a passenger locator form. I then collected my luggage, picked up my rental car and drove to the resort.
All in all it was stress free and well managed. Wearing a mask was the biggest inconvenience and I wouldn't want to wear one on a long haul flight. Considering how long most people were taking to complete their meals and their drinks, masks were probably worn for about half the duration of the flight which does throw into question the usefulness of wearing them.