I'm on a 5 too.
I'm pretty sure you'd only see slowdown if you loaded up your phone with a billion different fonts. As it is, all you're going to be doing is using a different font to stock, and probably use the same or 0.0000001% more cpu cycles. Not to be confused with themes which demonstrably cause some degree of slowdown - or so I'm told, not interested in themes, jailbreak is ALL about added functionality and shortcuts for me.
I do agree with your underlying point though, I don't go crazy with tweaks as I like my phone lean and lightning fast. Fortunately, I'm at the point now where I've got everything I want from jbing and my 5 remains as fast as ever with no perceivable impact on battery life