I'm experiencing lower than expected battery life on my 2015 Macbook Pro. I believe that part of the problem is that the High Performance GPU is being activated when not needed by some tab in Safari:

This is NOT a CPU related problem, as the CPU tab in Activity Monitor shows little activity:

How can I tell which tab is requiring the high-performance GPU?
[Update]
After closing all tabs, Safari is still requiring the high-performance GPU.