"Note that WebKit already partitions caches and HTML5 storage for all third-party domains." - Intelligent Tracking Prevention
Seems a pretty innocuous note but…
What this means is Safari caches content from third-party origins separately for each document origin, so for example if two sites, say
b.com both use a common library,
script.js will be cached separately for both sites.
And if someone has an 'empty' cache and visits the first site and then the other,
script.js will be downloaded twice.
So how much should we worry about Safari's behaviour from a performance perspective?