Premixing and sending mixed coins are two different phases for DASH.

Sending premixed coins is instant.

Mixing your balance can be done whenever you want. Speed will pickup with time, as more people use mixing and there is greater "availability" to mix with. At that point, when a lot of people are mixing, bloat and fees will be the issue. I think we already had a glimpse of the future when premixing with multiple rounds was implemented and the network got congested from all the people who were waiting to mix their coins for the first time. IIRC the fees were bumped a bit, or it was suggested that txs would go through faster with a fee - something like that.