Some Windows 10 Mobile testers have noticed that the OS has a lot of background activity that might slow down its performance. In a series of Twitter posts, Windows Insider leader Gabriel Aul has confirmed the issue is related to its Cortana digital assistant feature and that it is still in the new 10549 build.
"There are a couple of fixes in this build, but we have a big one coming still. Cortana BG usage can be super high in some cases."
The temporary workaround for this issue is a simple one, according to Aul: just turn Cortana off and the performance hit for Windows 10 Mobile should be taken care of. However. it looks like we will have to wait a while longer for a proper fix for this bug.