Progress as the zones rise past about 250,000 start to increase the monsters per zone back towards 10 and above, even with the reduction ancient maxed out. As the zone reaches 275,000 the monsters per zone reaches 16-17. The number of monsters grows as the zones pass by.
The slowing of the progress is intended to extent of the game playability. Additional zone progress requires almost constant action. Autoclickers are the best option. Running the game using a separate machine can eventually get the game past Ceus, Tomb Guardian and up another hero or two. The slower progress makes it very hard to reach the last achievement at zone 1,000,000. Hero Soul accumulation for ascendance also does not keep up above Skogur meaning the grind is slower and slower.
With Ceus, Tomb Guardian the ascendance heroes souls level should be around 1e9290 or so. Each cycle of ascendance naturally improves the active hero souls. The progress is markedly slower the earlier in the game when the lower levels such as Skogur and Moeru which are progressively harder. Ceus is simply many more cycles of ascendance. From Ceus, Tomb Guardian to reach The Maw requires above zone 700,000 and this gets worse at Yahciyi,
With the Maw the first hero upgrade needs level 110,000 and it get even worse. The second hero upgrade for the The Maw requires level 227,5000, the third is 350,000 and the fourth one is level 478,000. The fifth hero upgrade is at level 612,500 and the sixth upgrade is level 752,500. As you approach the last upgrade the progress rate falls to over 100 monsters per level. The progress now reaches several months per ascension.
The Yachiyi, Primordial Soul here is available at 1e71996 gold. The first upgrade is at level 157,500, the second is 321,000.
The game becomes even worse at at the Ace heroes. The costs and levels required are so far out there it may take several years or more of play to get there. This was set up this way to allow gameplay to continue for years. Ace Scout Rose costs about 1e107995 gold.
The Clicker Heroes guide has the fill table of heroes and their cost based in the known logic in the game, Based on the game experience the game is clearly now capable of well over 24 months of action.
At this point there is still maybe a month or more before I need to ascend again. This is because the ascend souls are still well below the current level by a substantial amount. The advancement in monsters is slowed as the timer for the 5s and 10s are now very short which makes for much slower progress towards reaching the final achievement of 1 million monster.