Usually with updates to software, developers try to address any bugs or issues that were present in the previous version. Unfortunately it seems that in the case of Android 5.1, Google has yet to address some problems that are still plaguing handsets like the Nexus 5 and the Nexus 7 which are no doubt rather annoying for its users.
The issues in question are related to memory leaks in which after prolonged periods of use, the devices start to feel sluggish due to the amount of free RAM remaining which is less than ideal. This is an issue that Google had acknowledged back in Android 5.0.1 and was actually reported back in 2014.
Apparently this has to do with Android’s memory management which is said to be inefficient, leading to apps and processes taking up more RAM than it should and not releasing them properly when not in use. This leads to slow performance and in some cases, apps crashing. Speaking of apps crashing, the camera crashing bug on the Nexus 5 appears to still be present.
This happens to users when a third-party app tries to launch the camera app, like Facebook or WhatsApp when a user tries to take a photo to upload it or to send it to their friends. That being said it is unclear when Google will address these issues, but in the meantime rebooting your phone will temporarily solve the memory issue, but it certainly isn’t a viable long-term solution.