I transfer through Heathrow a few times a year, normally when doing an ex-EU. My big gripe is the variability of it. Sometimes the people working on security appear to be awake and helpful, then you get through quite quickly (passengers ignoring the quite clear instructions not withstanding). Other times, the security staff are glacial, everything backs up, they pull every second bag for secondary inspection then vanish, they're rude and unhelpful etc.

On a good day, I don't find it noticibly worse than transiting somewhere like Amsterdam. Different, sure, but not that much worse. On a bad day, you just want to scream...

They need to fix the staffing problems, then sort out the queueing systems which don't always work well (eg long empty tensa-barrier queues with staff who shout at you if you try to duck under / suggest they might want to open it up), then it wouldn't be too bad.