The Clicker Heroes game definitely changes into a grinders game as the progress towards the last 8 or 9 heroes is slower than molasses. The game increases the monsters per zone. At zone 200K there are 25-30 monsters per zone and this increases as the zones increrase.
The idea is to make the final heroes harder to reach. Xavira changes the bonus per 25 levels tp 4.5x. The Ace Scout heroes all offer a bonus of 1000x. Each of the Ace Scout heroes are so hard to reach in the first place that even with 40000 hours played it may be unattainable.
The boss monster time also shortens as the zones grow larger. The effect is significant amount of ascend and redo the heroes upgrades etc. Some of the heroes have global upgrades.
The cycle of ascend and start over slows above zone 150K as well. The slow progress affects the game completely..