Neurons are very slow by comparison to electronics. They are still much faster than people.
But Thellim has been maintaining the Gift's advance instructions.
The first parts of Vanyel's tapestry to change, to go significantly out of bounds compared to the immediate past, are highlighted in Thellim's attention. Thellim's focus goes into trying to maintain them in view, maintaining the current result of the instruction she tried to give Melody's Gift; the Gift will not simply obey an instruction to freeze its results without ongoing concentration, they tried that earlier and it didn't work.
Thellim doesn't call out to Melody to follow the next step of the plan. Melody should already be acting. This has been rehearsed.