fq paces locally generated TCP traffic at the rate the TCP stack
computed for the socket. Data then leaves in an even stream instead of
line-rate bursts, which are likely to overflow the small buffers of
access points and switch ports in front of slow links (for example a
laptop on wifi).

fq also keeps a separate queue per flow and serves them in turn. When
the local link is saturated, a single bulk transfer can no longer delay
all other flows on the interface, as it can with the shared FIFO of
pfifo_fast.

Note: fq is meant for hosts that terminate TCP connections. On a router,
fq_codel may be the better choice: forwarded packets have no local
socket, so fq cannot pace them, while the CoDel part of fq_codel actively
keeps queueing delay low.