The British do crime series of only 3-4 episodes

They go for quality over quantity.

i used to watch some british crime. they were usually an 1hr:10min to 1hr:40min (without commercials). so they tended to be longer than american episodes

The british invented this 6 episode series, at least for us americans. Honestly it changed the face of television in the US...You have to go back to like 2005 but Americans had no idea how to do a 8 part limited series mystery until the Uk came in with Broadchurch..that blew our little minds.

It's a good format for adapting a novel when a film is not long enough.

That's how long most shows should be.

It's funny as I remember thinking the exact opposite when people were trying to get me to watch 24. As an English person , All I could think was ' omg, 24 episodes. Are you kidding me?! ' . I did eventually watch it, and loved it.

That's definitely not a bad thing.