If Messages is starting up at the same time sa the computer then start it separately.
It will "read" the speed it can see at this time and "write" it to the .plist involved.
At computer Start up the Computer is busy making connections itself and uses a large amount of your Bandwidth distorting what iChat or Messages can "See"
Do a Speed test and compare this to what you think you should be getting.
AS you say it can pay to try a router reboot if the speed looks a little low.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously