Metal can cause weird RF signals, but mostly as a result of reflection. You can think of it in similar terms as sound. In a cave you're voice bounces off the walls and reverberates, effectively smearing the sound. RF signals will experience the same thing. The signal might be loud enough, but completely unintelligible.

And that’s how your iPhone became a tricorder

Better signal strength does not directly correlate to better speed. Even if the signal was somehow better, the latency would be increasing deeper into the cave, which would drop the speed.