When I visit an http page for the first time and then exit. When I re-enter after 2s, I see that the pages are displayed faster. webkit's net threads are caching them? Where should I find out and how long are they cached for?
Does webkit store http data?
Browsers have had various layers of HTTP caching for decades now.
Yes, they generally cache resources on disk for later use.
But to answer "how long" depends on the server, the resource, system load, etc.
HTTP servers can request a resource be uncachable, cached for 5 minutes, cached for a day, cached for a year, etc.
Caches are - by definition - an opportunistic performance improvement but should not be seen as any sort of guarantee.