It's my understanding you don't even have to have OS9.x of any size to run OSX.1.2. In other words, OSX.1.2 can be installed and run without Classic.
How ever, If you want to run any OS9 application while in OSX, you will have to have OS9.x.x installed either on the same partition with OSX or on a separate partition and it will have to have the extensions needed for that particular application.
I have a partition with OS 9.2.2 and all the PPC applications I've used for years...then I have another partition with OSX.1.2 and have selected the OS9.x partition as my Classic choice. Since several PPC applications cannot be run from the OSX environment, I can then reboot into the OS9 partition to run those apps.
Startup time in OSX should not be affected by anything in OS 9.x, unless you have selected your Classic environment to start when you boot OSX...you do know this can be selected/deselected in the System Preferences Classic Pane?
Hope this helps,