How is mobile page speed calculated?

I ran a site speed test here and have two questions:

https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fwww.lisee.co.nz%2Fcollections%2Fjeans%2Fproducts%2Fmedium-wash-mid-rise-skinny-jeans-with-ripped-knees&tab=desktop

First. How is mobile page speed calculated? Is it intentionally slowed ddown to emulate a 4g (or similar) netowrk?

Second. If this is not the case, then why is the Shopify theme that I'm using perform so incredibly poorly?