It was two Dual-Core 2.5GHZ CPUs. I have one sitting under my desk right now. ;)

Pretty advanced for 2005. Four 64bit CPU cores, 16GB DDR2 RAM, and liquid cooled. It's still usable today 17 years later and it could work for 90% of what I do on the computer. It draws nearly 1000 watts under full load tho....

1000 watts, damn.

So what can’t it do? I’m guessing modern websites struggle on it

Yes, websites with heavy JavaScript really slow it down. The newest browser available for it is based on a very old version of Firefox.

https://github.com/classilla/tenfourfox/

Youtube is extremely slow, and it can only play back 360p or lower video smoothly. There is no hardware h.264 acceleration. “New” Reddit is also very slow. But old Reddit loads fine. Hacker news is very fast, loads as quickly as a modern computer.