Black screen for about a minute after the startup sequence
It may just be because your phone cannot handle marshmallow.

You have a Nexus 5 (released in 2013), therefore when you bought it, it would have been designed for Kitkat. The phone has now been through two other major os updates (lollipop and marshmallow). Because these updates are designed to work fluently with newer models, older phones are expected to see long loading times.