It is down to how you use your hero's skill points and what gear you equip your hero with.

I have found you can get better speed up bonus by carefully selecting gear rather than using skill points.

Here's an example. To unlock the Savages of Utgard takes 220 weeks (1540 days or 4.2 years)

I then equipped my hero, activated a 10% boost and selected Inga the Wise. This gave me a total boost of 240% 

This then greatly reduced the upgrade time to 77 weeks 5 days saving 153 weeks.

Then with your clan help it could be as low as 57 weeks.

The starting point for any build or research is high,but it can be greatly reduced,by concentrating on acquiring the skills, and other things that do this first.

Yes,you can buy time boosters but there are other ways of indirectly getting some.

If you are in a large clan, and your clan mates are doing their part they can reduce the times as well.

Picking the days you start processes is also beneficial, especially if your kingdom has won the kingdom event.

They dont come free, but second  builder and prophet are useful ,two builds or research done consecutively also save time.

Utimately, there is the use gold option,  for an instant finish.

True, like WDYWTNF says, pick a day to start the upgrade with  a 10% reduction on it. 

I try to go on a boosts fast and not use boosts over 3 hrs for a month before taking on a big upgrade. 

I also finish off level 6 rss tiles with only a few k rss left on them as they down field a small boost.

Then I only apply the boosters when there is a 10% added bonus to the boost though you never know which day that will be. 

If you're lucky there will be a maintenance break and you can be rewarded with 100 3hr boosts worth 12.5 days. Not much but better than nothing.