When your miner first connects to the pool, the default Difficulty it will get is currently 8190. After a few shares, the pool will attempt to adjust the Difficulty to match the hash rate of the miner.
This means that if you point a low hash rate miner at the pool, it may take a long time to find a few shares and have the Difficulty adjusted down. There is also a minumum Difficulty on the pool of 442.
If you have a low hash rate worker, you can set the starting Difficulty on the Workers->Management page. If your miner is 570GHs or less, then the minimum Difficulty 442 will mean it will always average less than 18 Shares per Minute. e.g. if your miner hash rate is 57GHs, it will average 1.8 Shares per Minute.
Shares per Minute
The pool adjusts your work Difficulty so that a typical miner should find, on average, 18 Shares per Minute.
While this number is somewhat arbitrary, it does mean that over an hour, the sample of shares sent to the pool will be large enough to get a good estimate of the miner's hash rate. At 18 Shares per Minute, your miner will submit, on average, 1080 shares per hour.
Does Worker Difficulty Matter?
For this explanation, lets say that everyone on the pool has miner type SuperMiner10 that the pool sets the Difficulty to 10,000
When your miner is mining at 10,000 Difficulty, it means it sends shares worth 10,000 to the pool as proof of the amount of work it is doing over the time between submitting each share to the pool.
1 Difficulty is 232 hashes or a bit over 4 billion. A miner is expected to find - on average - a 1 Difficulty share for every 232 hashes it does. If the miner is mining at 10,000 Difficulty, then it is expected find - on average - one 10,000 Difficulty share for every 10,000 x 232 hashes it does. After sending many 10,000 Difficulty shares to the pool, each 10,000 Difficulty share sent proves you have done the work equivalent of 10,000 x 232 hashes. Also, you can't fake it.
So e.g. if you have 2 miners sending 10,000 Difficulty shares - then you would send - on average - twice as many shares as another person with only one miner. Thus when rewards are calculated, you'd expect - on average - to get twice as much reward as the person with only 1 miner.
Now, if you change the Difficulty, it's expected to NOT affect your reward. The relationship between the Difficulty and the expected amount of shares is linear. If your SuperMiner10 is mining at 20,000 Difficulty, it will find - on average - half as many shares over a given time period vs mining at 10,000 Difficulty. So since each share is now worth twice as much, the overall result will be the same - half as many shares worth twice as much each.
If your SuperMiner10 is mining at 5,000 Difficulty, it will find - on average - twice as many shares over a given time period vs mining at 10,000 Difficulty. Again since each share is now worth half as much, the overall result will be the same - twice as many shares worth half as much each.
Of course if someone gets a 'new' SuperMiner20 that hashes twice as fast as the SuperMiner10, then they also expect to get twice the reward. The pool will set double the Difficulty so they find roughly the same number of shares as the slower miner, but each share proves you've done twice the work of the slower miner.
And lastly, before anyone gets some weird idea about trying to earn more by messing around with the Difficulty ... Nope you can't, coz people thought about that long ago and ensured you can't do that with Proof of Work.
Why Does the Pool Do This?
The pool doesn't know what hash rate your miner is actually doing, but shares do prove what equivalent hash rate your miner is doing. Otherwise a person could mess with their miner and get it to say it is mining at 1000 times faster than it really is. Again shares do prove what equivalent hash rate your miner is doing.
Now-a-days with Asic-Boost in most miners, even your miner doesn't know what equivalent hash rate it is doing, it must use the same process of estimating the hash rate based on the Difficulty of the shares it finds, otherwise it would report a much lower hash rate.
For details about how the pool does hash rate calculations see Help->HashRate here
If you wish to read up more about that Proof of Work, google 'BitcoinProof of Work' or 'BitcoinPoW'.