At the moment, initiative changes seem to be retroactive since the start of the battle - casting Haste mid battle with a decent number of air shards results in the recipient unit getting multiple turns in a row. Casting slow with multiple water shards mid battle may well result in a unit not getting a turn for a very long time.

I would suggest that initiative changes should only start calculating the new speed from the time they are applied, rather than applying a catch-up as if the unit had had that initiative from the start of the battle.