If you're actually using a 'strong enough' hash to prevent easy cracking if your hashed password database is leaked then you're doubling the server load which can be quite substantial in some cases.
Because you are changing the daabase schema to introduce a stupid version field to store "normalized" passwords rather then just doing the check twice on mobile platforms.
Hashing takes a lot of CPU time. And btw you don't even need to change the database schema. You could encode the version in the password field itself. Django does this and it works great
The database would contain existing passwords without normalization.
You also you have to hold the unnormalized password.
Super silly to do that to save a few processor cycles on login.
It's amazing how much misinformation is in this thread. You should do further reading on password hashing and rethink whether you really have to store two different passwords...