Consider the X.0 to be the base of each expansion.
2.0 is the base of A Realm Reborn.
You then have all of 2.X to get through, 2.1, 2.2, 2.3, 2.4 and 2.5. In total, all of that content combined is almost as much as another 'base game'.

Same goes for Heavensward 3.0 base and then 3.X patch content.
Stormblood's 4.0 base and 4.X patch content, etc.
Although 3.X and 4.X aren't as much as 2.X

If you're rushing to get to Heavensward, you're gonna have a bad time. A lot of the 2.X series is decent content on it's own, but a lot of it is also tedious fetch quests.
They're actually trimming these quests down in patch 5.3 which comes out in about 3-4 weeks.