This could be part of it. The shift to mobile computers which are necessarily wireless which means a random round trip time due to physics, which means TCP back

This could be part of it. The shift to mobile computers which are necessarily wireless which means a random round trip time due to physics, which means TCP back-off. That, combined with the tendency to require innumberable external JS/CDN/etc resources that require setting up new TCP connections work together to make mobile computers extra slow at loading webpages.