People will dive into stuff like the sitemap (XML or HTML), robots.txt, page titles, meta descriptions, dupe content etc.
But for me, the first thing I look at when doing an audit is this:
Think about it, the moment you enter in a website address for the first time, you experience the speed at which the homepage loads up.
That’s a good place to begin with because it doesn’t matter if you rank #1 for a very competitive term – if your site is too slow to load up, people are going to bounce back into Google and go to the next site in the list which is your competitor.
You can’t afford to lose people so easily.
Naturally you should test it on other devices, internet connections and different browsers (for consistency). If the trend is the same (ie. a slow loading site) you need to fix it.
How you do it depends on your situation: lots of images, lots of scripts loading up in the background, a slow hosting server and other factors all affect how fast your site loads up.
Install Google Webmaster Tools if you haven’t already and start getting an idea on crawl rates/errors associated with your site. It’s a good starting point to getting your site to load up so much faster.
Simple stuff that can make all the difference to your conversions.