There will inevitably be differences, especially between 12.4 classic AMD and 17.x AMD in high speed architecture. Key difference is that 17.x AMD reports separately the SSL connections setup and TCP handshake times. These are not a part of the page load time. Especially in cases where session setup were done in advance (predictively), differences may be large.
More specific answer requires looking at concrete case, based ona. packet trace, so opening a call in Support looks necessary.