You may not be deleting things you don't need you may not be de-fragmenting the disk. You are doing more things. Newer software needs a bigger engine. Where I live, near Bluebird lane, it's too far from the phone box to get any improvement. If you can upgrade your Video RAM, you're in good shape.
More fonts mean slower system.
More files in the extensions and preferences folders mean slower system. Don't just go yanking things, but try to move the unessential things out. Extensions should be moved using the Extension Manager control panel. Preferences can often be deleted. If a preference is needed, it will be recreated by the original application.