If I'm on an old Mac and I just want the basics (web browsing, text editing, multitasking) to run a lot faster and I don't mind losing out on bells and whistles or temporarily disabling unneeded background processes, what processes would this be and how could I enable/disable them with a simple shell script?
I suspect there is not much you can do here. This is because most background processes only run when they are needed (see "On Demand")
Best to look at Activity Monitor to see if there is anything taking up CPU.
My guess is for text editing and multitasking you will have no speed issues.
For web browsing the browser and especially add ins will take up CPU and memory thus requiring swapping.