Things drift, heat-up, wear out, excetera... so a perfectly balanced pack after day one doesn't really exist.  That's what a BMS is for... it makes up the little differences in the pack, and prevents bad scenarios.  

However, to compensate for an otherwise fully charged pack where one set of cells needs to catch up, the BMS handles this by reducing the current to the entire pack, and shorting the fully charged cells slightly, so that the balancing current only travels through the cells that need it.  A similar tactic is applied to discharging in that once one parallel set of cells is fully discharged, the BMS will 'open' the entire pack.  Ride over, even if you had 25% or more in the remaining cells.  

Ultimately this BMS protection kicking in is also why 2 out of 3 cells in a 'dead' surplus battery are usually fine and therefore is the reason we can build these packs for cheap to begin with... but I digress.

Bulk charging for a well balanced pack can happen in under an hour.  Balance charging can literally take days.  For example, my BMS is rated for 15A charging current, but once we enter balance charging land for leveling the cells,  it is a painfully slow 56mA.  

The better balanced the pack, the better the BMS will perform, and the happier you will be.