I nearly missed a connecting flight in Heathrow earlier this year, because it took them thirty minutes to get seven people through security. I kid you not - I was seventh in line at security[0], and they were only allowing one person at a time to unload the contents of their bag onto the conveyor belt. They had five or six agents all "working" there, and a peak throughput of one passenger every three minutes.
[0] International flights connecting in Heathrow have to go through security again, even though they've already gone through security at their original point of departure.
That's not terribly uncommon, or particularly unreasonable -- security standards and implementation vary significantly in different countries, and in as much as security screening is useful, you'd want to redo the screening with the local standards, unless you have separate facilities for international flights arriving from countries you know do a good job and other countries, but the customs/passport control section is probably shared among all international arrivals.
Nowadays there's very little difference to the security procedure between the two, and I'm always surprised it doesn't hurt Heathrow more as a hub for ex-EU passengers.
That said, in the UK case at least, it requires a change in law, so it needs to become enough of an issue it's worth some at least small amount of political time.
I fly internationally very frequently. I'm aware that it's not uncommon to have to go through security when making a connection. I'm saying that Heathrow's security is incredibly slow.
So apparently Germany trusts some countries' airport security, but the UK doesn't.
They apparently don't trust Singapore. I was on a flight from SIN->JFK that had a stop in FRA (we got off and got back on the same plane). We still had to go through security.
JFK->SIN though we could get off and didn't have to worry about any security or immigration.
Mumbai has always been a breeze every time I've flown there, compared to Heathrow.