Originally Posted by stifle

Well it depends on how you're arriving at T5. By train or tube it's maybe 5 minutes more, then it's around the same length of time to get to the departures floor at T1 and T5. At T1 you have the photo checkpoint and fast track security, then 5-10 minutes to the lounge depending on how busy the second photo checkpoint is, then 1-2 minutes from the lounge to gate 8. At T5 you go to the photo checkpoint and fast track north security from where it's all of 20 seconds to the north lounge and then maybe 3 minutes from the lounge to gates A1-7, unless it's A19 in which case 7 minutes. The second photo checkpoint is built into the boarding process.

Overall it seems like basically the same amount of time, but the one variable is conformance because at T5 if you scan your boarding pass at security a minute after six you'll be refused.