Isolate High Variance vs Low Variance Sections of Signals
What you did is the reasonable solution.
From here you can do 2 things to mitigate your issues:


Computation Efficiency
You can use online calculation of the Mean and the STD.
Remember when you move one sample to the right 24 samples are shared.
Hence if you take advantage of that you'll get a performance boost.
Threshold
Use adaptive threshold per signal if your classification of the STD is relative. Namely per signal you want to find zones with high variance relative to other zones in the same signal (And not an absolute measure).
You could use statistical properties of the STD of the signal like Mean / Median or even use adaptive Threhshold method like Otsu's Method.