I haven’t attempted to listen to Slayer in a long time, but they were always too ugly for me. Maybe things have changed. After all, I like early Megadeth now. They only have one consistent album though. And they’re definitely a band that makes me glad I don’t share your compulsion to plow through entire discographies.
I watched a youtube video where, if I’m remembering right, somebody reviewed every Megadeth album after listening to them all in a row nonstop. He mostly only liked the early albums. That judgement is probably correct, but I don’t think the format probably also skewed his opinions.

I rarely have any issues with the runtime on thrash era Metallica songs. Actually, I’ve always thought that, at least comparing it to other tracks of the era, For Whom the Bell Tolls sounds like it ends 2/3 of the way through the track.