Upgrades take a lot of time
Many of clash of Clans upgrades take a lot of time. Some upgrades might exceed one week. A player will have to wait for an upgrade to finish before they can upgrade the same structure again.
Upgrades are exponential
In the beginning of the game, upgrades are finished so fast, but as a person progresses, the upgrades start taking much more time.
Builders are limited
The number of builders in Clash of Clans are very limited. The maximum number of builders that a player can get is five. There are so many upgrades in the game and this is why the game can take so long.
Resource requirement increase exponentially
The resources required to make an upgrade increase exponentially with each upgrade. This makes it harder for the player to upgrade their buildings more as they progress in the game.
You get less resources when you upgrade the town hall
When you join a new league, you will get less resources from enemies with weaker town halls. The game was designed this way to ensure that players will keep playing it for a long period of time.
You get less resources when you join a league
When you join a new league, the enemies you attack will become stronger and so it will become harder for you to get more resources. The walk around here is to keep losing battles until you join a lower rank league. (See How to Protect Resources in Clash of Clans?)
Gems are limited
Gems can push the progress for upgrades, but they are a very limited item, unless a player decides to buy gems from the store.
The required gems increase exponentially
With each completed upgrade, the amount of gems required increases exponentially. For example, an upgrade might require 20 gems to complete then the next might require 80 gems. (See How to get free gems in clash of clans?)
The army requires some training time
A player can’t make many consecutive attacks in COC, as they will have to wait for the army to be ready. This waiting time slows down the process of resource gathering, and in turn the upgrades.
There are too many upgrades
The game has too many upgrades and a massive amount of resources is required for a person to complete them all.