Couple things to consider here in terms of length of fight.
-There are only 59 out of 81 possible casts of riptide.
-Velen’s was used 5 out of 7 possible times.
-Gift of the Queen was only used 6 out of 11 possible times and used poorly. You want to always try to hit the maximum 6 players it can hit.
-Cloudburst Totem was only used 9 out of 17 times.
-Ancestral Guidance was never used even though the shaman is specced into it so thats 0 casts out of 5 possible uses.
-Healing tide totem was used 1 out of 3 possible uses.
-Spirit Link Totem was never used so that is 0 out of 3 possible casts.
When you look at the mana of the shaman vs the health of the boss you can see that there is a huge issue here. Your shaman has near full mana the whole fight. I believe aiming to have the same same percentage of mana or a little above the boss health percentage is a good measurement.
All in all, I think your resto shaman just needs to try to cast more often. Get more usage out of her CD’s. Try to cast gift of the queen while Cloudburst Totem is up. The healing on CBT has a high overhealing ratio as well, try to time it a little better so the overhealing isn’t so high.