Having now played nearly 2000 games on Lichess, I'm pretty happy with their ability to weed out cheaters. I've very rarely had cause to suspect my opponent wasn't playing fair. Maybe once or twice someone with a lower ranking than me seemed to be a bit too good --but then we all have our flashes of inspiration, as well as our off days.
However, on a couple of occasions, I've logged onto the site to see a notification that I've had some ranking points restored, as a previous opponent had cheated. But, unfortunately, Lichess doesn't tell you which opponent or which game, so I'm still none the wiser.
PS: Agree with you about the people who run the clock down, when they're losing. Really annoying. When this happens, I usually switch to another browser tab and read intarwebs instead, switching back to the Lichess tab every 5 mins or so, to see if they've made a move yet.
Yeah, unless you want to play a longer game than a 5 minute blitz.