Hello - next month I will be flying into Heathrow and taking the Eurostar to Paris, which I have read is not implementing EES until fall. I then will be flying back to the U.S. out of CDG then without having registered yet with EES. Will this be a problem? Has anyone experienced delays due to EES registration on departure? I am assuming I should arrive at the airport 3-4 hours before my flight departs, or should I plan even more time? Thank you!
EES is underway now. ETIAS (a different thing) is delayed until fall.
You might go through EES when you clear the border in London getting on the Eurostar, see this info: https://help.eurostar.com/faq/rw-en/question/What-s-the-EU-s-Entry-Exit-System-EES Follow the arrival times listed on your Eurostar ticket, you may need to be at the train station 90 minutes (or more) ahead of departure to get through immigration (exiting UK, entering EU). Not all EU airports have fully implemented the system and are still manually checking passports.
CDG always takes time to get through, so 3 hours minimum ahead of your flight.
And just to be confusing there's a different requirement for the UK, be sure you get an ETA for your UK arrival, purchase on the official website https://www.gov.uk/eta