There are known problems with El Capitan and the My Cloud. Seems that for some Mac users Timemachine is slow. And that slowness is made worse if one is backing up over WiFi. Here are some prior threads that discuss these issues. Don’t know if there is single set workaround or fix though.
http://community.wd.com/t/support-os-x-el-capitan/96744
http://community.wd.com/t/slow-data-transfer-aoole-time-machine/150266
http://community.wd.com/t/time-machine-backup-how-slow-is-slow/95629
