I've been wondering this for a while. If I set unique permissions on a lot of lists and items in SharePoint, will that result in performance issues? At what point would it become a problem (100 items, 1,000 items, 10,000 items, etc)?
It depends on farm configuration, server processors, memory utilization and of course your SQL Server setup and machines. But leaving that out of the question there is effect on performance when you have item level permission enabled. As a rule one should not architect a solution where item level permission is needed.
But more than once I've come across solutions where every new item gets its own unique permission. On a 3-tier farm in 2010 I could experience an effect when the number of items exceeded 1000. Even if I had limited the number of items returned at once. It shouldn't take longer than a second retrieving 30 items, but it took 10 seconds (also depending on large pictures on each item).
In another fully redundant shared services farm (2010), we didn't experience any issues with 8'500 documents in a single library where every item had unique permission. Instead the search crawling got very slow and used 90 minutes on a full crawl (nightly). The following incremental crawl used an hour to update permissions. The following incremental crawls used 25 minutes even if nothing changed, just by reading and comparing permissions.
I've come to the conclusion that unique permission will effect performance one way or the other.