The UK was never part of Schengen, but it was part of the single market. So while passport/ID controls have always been performed on the UK border crossings with other EU countries, customs controls had not – until Brexit.
I am not aware that customs controls are made in the passport area, at least not between London and Paris. It is done at the exit of the terminal, in a non blocking way like in UK airports (and always was, they were already looking for illegal products before Brexit).
The only appreciable difference I see a need for a border guard to physically inspect and stamp passports, which I suspect slows things down. I guess EITAS checks in the future too.
Once EES comes into effect the stamps may not be necessary anyway, but much like fusion, it's perpetually 6 months away.
The main difference is that pre-2021 UK travellers simply had to show travel documents whereas now they have to get them stamped.
On the surface, it isn't clear to me why that makes much of a difference in service time, but the CEO of Eurostar pointed a finger to these changes in January: https://www.bbc.com/news/business-64390979, claiming their trains carry 30% fewer passengers due to station bottlenecks.
And yes, they claim the EES checks should help speed this bottleneck.
It's total nonsense. These trains are running now with the same amount of space with no issues - it's only 4 trains a day, with the closest one being 2 hours apart. You can easily check a trains worth of people in 2 hours, brexit or no brexit (and I am sure eurostar would have changed the timings if in some crazy world it did require more than that).
What the article should say is 'given the requirement to do full passport and security checks'.