I want to use about 10-15 icons on my website. Should I use CDN or download all of the icons separately?
Which will be more fast CDN or using separately and what should be preferred?
Very difficult to answer this as it requires more information about the size of the icons you want to use, the size of the full icon set, your websites host transfer speeds and the speeds of the CDN.
Best way for you to make a call is to test it yourself, include the 15 icons and use the Network tab in Developer tools to see how much time it takes to load the icons. Then switch to the CDN and check that. Best to do each test around 20 times on different connection speeds to get a good enough average.
But remember, one of the main benefits of using a CDN is that hopefully the user will have already come across the files and have them cached, which means they won't have to go through a HTTP transfer.
My particular answer would be go with the CDN unless the difference in size is large.