If transactions were close to instantaneous, it wouldn't be an issue. Buy in USD -> Pay in Bitcoin -> Receive Bitcoin -> Convert to USD needs to happen in under a minute.

However.

Those transactions frequently take a substantial chunk of real time in which time the value fluctuates more than a currency normally would.

It's almost impossible given the Bitcoin network can only process 3 transactions per second.

While Bitcoin fans say it's both, their behavior is usually divided between using bitcoin as a form of value sink or value transfer. The former is good for an asset, the latter is good for a currency.

Right now, the volatile behavior of bitcoin shows it being much more of a speculative asset than a currency.

Currently, Bitcoin is used as a store of value not so much for day to day transactions. It's still evolving, its software, and it's upgradable.