One of the rules of cryptocurrency mining is that block should be found in certain amount of time, which is fixed. This guarantees the stable and predicted money volume generation. However mining itself can be compared to lottery. While in the real lottery a chance that you win is fixed and miserable in mining it can be adjusted to guarantee the the period of block discovery.
Difficulty influences the workers job submit rate(Acceptance rate). Ideally if difficulty becomes 2 times bigger e.q. 128->256 your worker should start to submit 2 times more rare. However mining performance in term of profit should not change, because your shares (and reward) are calculated using both number of submitted jobs and their difficulty (something like subm.job*diff). Basically if worker is weak and difficulty is big, then it is more like solo mining. Mining in pool however has different goal, right. So bigger difficulty is not the end of the world, but can affect the daily performance of your worker. So one day you'll get less and another day more. Be ready for that.
Rejects however can influence the performance. The problem is that your luck in finding shares depends on difficulty, but rejects depend on network connection, pool and cgminer settings. What does it mean ? Well suppose your miner have finally found a share with appropriate difficulty. But because network connection has problems it can not send it in time and gets a reject.
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Rejects however can influence the performance. The problem is that your luck in finding shares depends on difficulty, but rejects depend on network connection, pool and cgminer settings. What does it mean ? Well suppose your miner have finally found a share with appropriate difficulty. But because network connection has problems it can not send it in time and gets a reject.
Useful links: