CDG immigration is much easier to clear than LHR. CDG I've had a max of 12-13 minutes. Even with a fast track ticket from flying in Biz, it can still take you 20-30 min at LHR. There's people who have had 3 hours in between flights on MR's to LHR and have come close or missed the return since Immigration/Customs at LHR is a lengthy process.