This was because that was my old machine a pre-iCore pre-HD graphics machine that will only do OpenGL v2.0 max, and has only 4GB of RAM. So I think it needs to download more compatibility stuff than newer machines, in order to run on the ancient Mobile 4 Series graphics in WebGLv1 mode.
I just tested on a brand new MSI gaming notebook with a GigaBit wireless, 16GB RAM, iCore i7, nVidia GTX1060 w/ 6GB … and SketchUp Web loads into Chrome in 12 secs having never ever been loaded before,… then reloads in about 7 secs.
EDIT: just reloaded in 5 secs.