from Seattle, Monday, May 20, at 6:40 am. I am happy to report that there were no paper Border Force Landing Card immigration forms to complete AND the immigration e-gates were open for business for US citizens! Sorry, I did not think to check for other non-EU nationalities - the line was moving to fast!
Edit: Now that I think about it I do not remember seeing a non-EU line.
There were several hundred people snaking between the stantions ahead of me, but the line moved along at a normal walk. There were 23 e-gates available with one or two people waiting to use each e-gate. Just slide your passport in the slot, get your photo taken, and the gate opens and you are on to baggage/customs. It took less than a minute.
As a side note I jumped on the underground at terminal 5 and as expected, it was a packed train to Picadilly Circus.