We can go a long time, but in the end it will be about explaining the why of each performance guidelines posted above, but with long technical explanation.
Note that every content/performance optimisation guidelines are the same as ours, for technical reasons too long to explain, but Feel free to dive in, and read all oculus/arm/mali/apple technical optimisation guide 
As said above, we try our best to make it as fast as we can, but sometimes there’s hard limit, here hardware and browser platform.
Please understand that the difference between unity, native code, optimized by device exports, with long export time to optimize specifically and access to all device specific API optimisation, Versus loading in a browser, whatever the device is, with the fastest loading possible is a big thing.
Unity long export time allows for instance texture GPU compression specific to each device, scene specific optimisation to compute exact texel ratio to lower texture size without visible impact, triangle to screen ratio to decimate accordingly, shader specifically compiled for the content and the device, etc. )