Get yourself in a clan and start hitting checkpoints in challenges and earn a cr@p load of boosts.

Also with a level 21 town it means your clam members can reduce your build time by 25%.

Then use these boosts to cut down on your building time.

Also the right hero skill set and gear helps a lot. You just have to find the right gear and there is a lot of good standard gear to help reduce these times.

If you think build times are long, try resetting your hero's skills and unequip them then see how long the building/knowledge upgrade times really are. You're looking at 2-7 years for some e.g town upgrade, tier 7 troops unlock.

Lastly if you could race your town from level 21 to 35 in a few months then where is the challenge. You would be left upgrading your stronghold and those drafts take 6 months.