I measured an LCP or 3.5s + significant layout shift.
The images are poorly optimized jpegs, instead of WEBP/AVIF.
The start page takes a cool 6MB.
A games page clocks in at around 12MB before the video starts loading including a whopping 4MB JS.
None of the links appear to utilize preloading and it's and old school multi page app, so navigation takes a long time.
I don't have a way to measure it, but subjectively it performs worse in the Steam client than in a browser.