Leveling actually is really fast compared to most games. I've been able to take a new character from level 1 to 70ish in just a few days of casual playing and all the big mods out there actually reduce the amount of experience gained simply to slow progression to a normal level.
As for leveling, you can always play on Veteran mode for Normal and skip all the Bounty Board quests. I generally hit level 55-60 before reaching Elite and I don't skip any side-quests nor do I farm that much. Once you reach Elite and Ultimate, it's a better idea to skip most of the trash mobs and just go for heroes and big clusters of enemies.
However, if you're having a hard time leveling and keep making new characters because you get bored, maybe your builds need some tweaking to help you clear mobs. Basically, everything in Veteran should die at the slightest sneeze from your character - if you're having a harder time than that, your build is missing something. Pure poison casters are my favorite and I ran from Veteran to Ultimate without dying a single time and my poison Witch Hunter is the build for my only two level 85 Hardcore characters - the rest died in the 70s. Most run of the mill builds can clear Veteran without any issues, but if you spread your points too thin, you're going to have a hard time. If you sink stat points in the wrong stat, you're going to have a hard time. You can always feel free to post your build(s) here and we can try to assist.