With Lollipop comes the complete destruction of the recents app. It is now pretty much unusable since it is now more of an overview screen than a recents view screen. The multitask button should only show open apps, instead it shows running apps, plus apps that have ran in the past. Over time these apps stack up and include every app that you have ever run. This quickly becomes totally useless.
Recently for root users makes this feature usable again. You can actually set Recently to only show running apps in the recents view app. The aggressive setting is often too aggressive however and may kill off apps that were open in the past several minutes. It is suggested that you configure it to kill off apps that have been opened in the past few days. There is an option to add a clear all button to your recents app, along with a few other advanced settings. This app is for root users only. If you are rooted grab this app from the link below.
via XDA