With Ethereum you need to consider 2 charts. The difficulty and the hashrate.
Looking at the overall hashrate before and after fork,
https://etherscan.io/chart/hashrateIt looks constant
Looking at the overall difficulty before and afterfork,
https://etherscan.io/chart/difficultyYou see the huge drop you were referring to, its because the ice age was reset and block times were reversed back to 14 seconds from the 20 seconds. So the difficulty dropped, hashrate stayed constant, price constant, mining rewards lowered and profitability fell a little.