Just a bit of commentary... Note that it's not "one process per tab". Chrome has "process-per-site", and "process-per-tab" (where a "tab" is not one tab, but a bunch of related tabs.)

https://sites.google.com/a/chromium.org/dev/developers/desig...

In general, IPC is slower than threading--Every time you switch processes you have to flush the TLB, as opposed to switching threads, which only has context switching overhead.