Okay I think I figured it out. The setting you want to look at is
r.VT.MaxUploadsPerFrame (default = 8)
Increasing this to e.g. 32 makes the VT tiles load in a lot faster for me.
This problem is further made worse by the fact that the pages load a lot faster in the editor than in build by default, because there is a editor-specific setting
r.VT.MaxUploadsPerFrameInEditor (default = 32)
so in-editor it seems like everything will be fine, but then in a build, everything loads a lot slower by default.
By the way there is also a third setting,
r.VT.MaxUploadsPerFrame.Streaming (default = 0)
which I guess applied to streaming VTs specifically, but I also guess the default value, 0, sets it to whatever value is set in r.VT.MaxUploadsPerFrame.
I really don’t understand how Epic thought they didn’t need to document this anywhere?
EDIT: One more thing, be aware you cannot specify this setting in any .ini files as far as I can tell. Only way to set it in a build is through “execute console command” node or C++.