Yep, weird that JS performance is not impacted though, only wasm. Anyway mystery solved!
Discussion
wasm has to be fully downloaded and parsed before being run. devtools being open often means the cache is off. this means the wasm file is redownloaded and parsed each time. JS doesn't have this issue. browsers, I believe, can start running the code while parsing it.
It depends. Sometimes the whole code must be downloaded first