Try changing the Zap Delay to zero: On-Screen Display * - MediaPortal Wiki
Why the default is 2, I don't know, but essentially it waits for 2 seconds before even starting the process of changing channel.
On the combo backend/frontend, that will change the speed to the same as Media Center, on the frontend only, it will probably drop it down to 3 secs.
mm1352000 - slow channel change speed is the number one issue for most people trying MP, yet this setting is rarely (if ever) suggested as the solution. Why isn't this either set as zero by default, or at least well publicised? The description in the wiki doesn't accurately describe what it does. Infact the Zap delay & Zap timeout descriptions are reversed (at best).
Justin