First, just give them 64 feeds and tell them about it on some form of status bar. The "updating your feeds 63/64" progress bar, but maybe yellow with "We're only updating 64 of your feeds. [a href="why-premium/"]Why?[/a]"
Also, you can choose those 64 feeds to update as a union of what they have subscribed to and what you're currently already having to fetch, thereby making the necessary amount of work for each new user theoretically less in the best case, and no worse in the worst case.
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You can make this much easier on yourself.
First, just give them 64 feeds and tell them about it on some form of status bar. The "updating your feeds 63/64" progress bar, but maybe yellow with "We're only updating 64 of your feeds. [a href="why-premium/"]Why?[/a]"
Also, you can choose those 64 feeds to update as a union of what they have subscribed to and what you're currently already having to fetch, thereby making the necessary amount of work for each new user theoretically less in the best case, and no worse in the worst case.