How to analyze website performance?

I'm working on a website which behaves right in desktop view, but when I load it on a mobile device it terribly slows down, the browser freezes. The interesting thing is that it rather has to do something with the size, not the device itself, because if I simply resize the browser window on my PC (or use any responsive web development tools that resizes the page to preset mobile device screen sizes) I can experience the freezing too. How could I analyze the performance of my website to get any idea about what causes the slowdown?