To be fair, it's still experimental according to the article, not even in beta yet. I wouldn't expect a smooth experience.
Safari uses WebKit which is known to be quite far behind Gecko and Chromium when it comes to support for the latest JavaScript functions, which I'd expect WASM to make use of. It will likely take some time to smooth out these issues.
Personally I have no interest in a web-based office suite, or indeed most web apps, but I can how they could be useful for others, so I see this as a good thing overall. I'd also hope it loads quicker on future visits as the components will hopefully be cached.