bug#31179: 26.1; eww leaves processes that slow Emacs

[Alex Branham <alex.branham@gmail.com>]

Eww is slow and makes Emacs slow when visiting a website with (for
example) gifs[1]. Which is understandable, given that a text editor is
trying to display a modern web page.

However, Emacs is slow even after killing the eww buffer. M-x
list-processes shows that several processes are still lying around. Is
there a way to make eww kill these?

Footnotes:
[1]  As an example: http://www.mostlymaths.net/2016/09/more-emacs-configuration-tweaks.html

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2018-04-11 built on mars
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-gameuser=:games
 --with-sound=alsa --with-modules --without-gconf --without-gsettings
 --with-mailutils --with-xml2 --with-x-toolkit=lucid --with-xft
 --with-xaw3d --with-imagemagick 'CFLAGS=-march=x86-64 -mtune=generic
 -O2 -pipe -fstack-protector-strong -fno-plt'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS NOTIFY ACL
GNUTLS LIBXML2 FREETYPE LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11
MODULES THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix