It's normal to have slow cast times like that. The first haste breakpoint is really low, and resto shamans don't primarily use haste as a means to speed up their cast times. Haste is there to give your HoTs/totems more ticks. And we don't need the haste for spellcasting anyway as we get it by the Tidal Waves buff on us.
For HW and GHW, you have the Tidal Waves buff to speed them up (created by a Riptide cast or - not so importantly - a Chain Heal cast). With Tidal Waves active on you, your cast time for the Wave spells drops to around 1.5s or so.
That's why you want to use Riptide as often as possible. Casting Riptide (or Chainheal, for that matter) gives you 2 stacks of Tidal Waves, meaning that two single-target spells you cast after that are affected by it. So, a typical "rotation" is to RT-GHW-GHW or RT-HW-HW. After those two spells, there's a break in which you don't have Tidal Waves active and RT is still on CD, so you'll either cast a slow heal or a Healing Surge (that one's always fast, and when a shamy's gotta heal a shamy's gotta heal) or do something else (like dropping a totem, or Unleash Elements/Healing Rain, or something).
Tidal Waves also applies a crit buff to your Healing Surge, so if you have the mana for it, you can opt to cast Healing Surge rather than Greater Healing Wave. The hps is higher, but it's more costly. Healing Surge is always fast, and with its high crit chance under Tidal Waves, it often procs a smart heal called Ancestral Awakening that transfers 30% of the healing to the poor guy who is sitting at the lowest hp in the raid.