Macbook air taking long to boot up (Over 4 minutes)
A typical issue causing long boot times is a faulty or missing selection of the startup disk. Go to System preferences > Startup Disk and make sure your Startup Volume is selected there.

Another thing which definetly helps to improve startup times on my MBP is running Onyx from time to time and have it rebuilt system caches, etc. (see tab "Automation"). Make sure to backup first.