Also please stop using distro releases of calibre. Most linux distros use some form of FeatureFreeze (and/or patch calibre in unwise ways), which means calibre is usually multiple years out of date. calibre is updated weekly.
calibre v2.x brought Qt5 with a headless Qt plugin that greatly improves memory usage when running the CLI tools.
Currenty calibre is on v2.30.0 which means you are 54 versions behind. (So just one year and a bit, then.

)Thank G-d the news recipes use the most updated versions available on the calibre website.