Only go on my personal experience a few weeks back. Came in at 1430 on a Wednesday, parked on stand 44 got to UK Border and hardly anybody else there, no waiting straight out (no bags) onto shuttle bus to hire car area. Left car hire at 1500. On return Monday morning got taxi and arrived drop off at 4.15 am - to beat the 5am rush - having dropped the car off the previous day (had been caught out some months earlier). Few people about, checked in straight away and no more than five minutes in security. Noticed around 5.30 security looked incredibly busy.
Over a year ago had to wait over 40 minutes at UK Border but that was around 2230 at night with a number of arrivals.
Regarding the easyJet Luton-Paris route an article in a national newspaper recently was saying that between 300,000-400,000 French people live in the London area making it one of France´s ´´largest cities´´ in terms of population - but of course don´t believe everything you read in a newspaper! Also a number of passengers must use the route to go further afield and escape the ridiculous APD charge.