I collected speedof.me download speed results for geckoview_example, Firefox Preview, and Chrome on two android devices, the Pixel 3 and the reference phone, Moto G5.
(Based on Kamyar's observations)
On the Moto G5, download speeds for Geckoview_example and Fenix are ~40% and ~50% of Chrome's, respectively.
On the Pixel 3, download speeds for Geckoview_example and Fenix are ~84% and ~94% of Chrome's, respectively.
Latency also appears to be lower on Chrome.
There is a short writeup on speedof.me regarding how the tests work:
It downloads progressively larger contiguous files until they take longer than 8 seconds to download. The timing for the last one is used.
I also run some tests where I modified network preferences.
network.http.max-connections doesn't look to improve gecko download speed.
This makes sense since from looking at what the site does it's fewer resources downloaded, but they are quite large.
I tried increasing others prefs that I thought might possibly impact this:
network.http.spdy.default-hpack-buffer (set to 4k on android)
network.http.spdy.push-allowance (set to 32k on android)
These didn't appear to impact the reported speed.
The resources I saw in the dev tools did come through via http/2.
If anyone knows of other configurations that could impact this, let me know and I'm happy to try them.