Because Macs are fucking slow.
Or rather, your $999 MacBook Air with the 8 GB of RAM, i5 processor, 40 Chrome tabs (ten of which are holding on paused YouTube videos, and one of which is in the middle of a GoToMeeting session), Spotify, Slack, Mail app, iTunes, Evernote, and every Microsoft Office app under the sun, that you haven’t rebooted in six months, is fucking slow. That’s just how they are. Try to do less stuff at the same time or get a more powerful computer.
“Right… but… it shouldn’t be slow, though?”
I hate computers, you know?