If one of many Safari windows loads from a slow server all other windows (loading from much faster servers or even the file system) will be just as slow. Just open a few (eg 4) sites in different windows, and pick a slow one: 4 blank windows. Wait 10sec, nothing will change. Then cancel opening the slow one and within less than a second all other windows will complete loading! It's especially obvious with ssl, as Safari is extremely slow on https even with fast servers.
reproduced this using a rate-limited apache. nasty.