I came to write exactly this comment.

The thing runs instantly. And that's in a VM in Javascript.

Office really chugged on the PCs of the time though. We can debate whether modern Excel actually delivers enough more value than historical Excel to justify being as more resource-hungry, thus slower to load, as it is. But historical Excel appears fast on modern hardware, even in emulation, because the CPU, RAM, and permanent storage have had 30 years to evolve since it was released. Contemporary 386s and 486s would not have been that snappy.

If this were a commercial project then I could understand the complaint.. but this is just a small, for-fun project and they have little motivation to put the extra effort into support for all browsers.

Freecell is actually a 32-bit Windows application running through a Wine-like compatibility layer called Win32s on 16-bit Windows.