Hitting the wall

I consider level 40 to be a wall of sorts for me. I find at this
point, the quest chains get longer, the dungeons take more time and
the going gets much slower.

1-10 should be your racial starter zone.
11-20 Silverpine Forest
21-30 Hillsbrad, Tarren Mill
30-35 Arathi Highlands and dungeon runs
35-40 Andorhal and Western Plaguelands
40-45 Eastern Plaguelands, Tanaris, and Camp Taurajo