WPT uses Python's http.client.HTTPConnection to send/receive WebDriver
messages. For some reason, on Linux, we see an ~0.04s delay between the
WPT server receiving the WebDriver response headers and its body. There
are tests which make north of 1100 of these requests, which adds up to
~44s.
These connections are almost always going to be over localhost and able
the be sent in a single write. So let's send the response all at once.
On my Linux machine, this reduces the runtime of /cookies/name/name.html
from 45-60s down to 3-4s.
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