If you're trying to download a clean OS X image, what better place than to get it straight from the app store itself? (It's legal after all!)
One issue I encountered is that my download speed was horribly slow, whether I was on wifi, or using an ethernet cable. I did some searching on google, and I seen that most people are able to reduce their download speeds from a few days, to only a couple hours at most.
I assume Apple just auto assigns you to a server, and if it's under heavy load, you may experience slow download speeds, however I have performed the following steps to fix the issue:
NOTE: Prior to taking these steps, note your original DNS in case you don't want to use these DNS servers later.
The Google DNS Method:
1. Open System Preferences -> Network -> (Choose your network adapter) -> Advanced
2. Click on the DNS tab and press the + button located below your default DNS.
3. Type: 18.104.22.168 and press the + button again.
4. Type: 22.214.171.124 and click on OK.
5. Press Apply and close the window.
Although it should be automatic, it would help if you unplugged and reconnected your ethernet cable (or disconnect and reconnect if you're using WIFI) - The DNS servers you're using are the ones provided publically by google, so don't forget to reset them
Go back to the Mac App store, and try redownloading your OS X image, and watch the speed go up!!
An alternate DNS that some people claimed to have worked is this one: 126.96.36.199 however it didn't work for me, but I won't rule it out as an alternative if the Google DNS method doesn't work for you.
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