Xpra supports lz4 and lzo compression if dev-python/lz4 and/or dev-python/python-lzo are available. Please add them to the dependencies (maybe introduce lz4 and lzo useflags?).
Note that at least version 0.7.0 of dev-python/lz4 is required (currently ~arch).
commit 41242054998465c31e550afb229ebdc6f785b467 Author: Michael Weber <xmw@gentoo.org> Date: Mon Feb 22 12:41:40 2016 +0100 x11-wm/xpra: RDEPEND on dev-python/pyopengl_accelerate (bug 572122), dev-python/pycups (bug 571978), dev-python/lz4 and dev-python/python-lzo (bug 571976). Thanks to thanks Fabian Henze for the reports. Package-Manager: portage-2.2.27 x11-wm/xpra/metadata.xml x11-wm/xpra/xpra-0.15.10-r1.ebuild