Re: reproducing locally - no it doesn't happen locally, even remoting into the server and using Chrome directly on the server. Have enabled debug and can't spot anything specific causing the issue.
Just today, the same issue seems to have started with one of my other version 12 sites - a very simple site with no custom development whatsoever. A secured "members" area, but it contains only stock-standard content pages with security applied.
AND I've just had a call from my hosting support guy, who tells me he's been contacted by one of their clients who is experiencing extremely slow upload times when using latest Chrome! (Nothing to do with a Kentico site.) So I think the issue seems to be something to do with the latest Chrome update.
My next step to try and pinpoint what is causing it is to create a completely clean site, no content in it or anything, and just start adding stuff bit-by-bit and and try and figure out the exact point where the delays start.