This being Mysql, I think you might have a memory issue. IIRC, Mysql is notorious for fetching all rows for a query, before it even starts to return the first one. If that's the case, 400000 rows of 200 columns is a lot of memory. So maybe your computer is just trashing about.

But perhaps they fixed it in a newer Mysql version. Or maybe there's a setting to tweak Mysql, so that it doesn't do that.