@elevated_subwaysIt sounds like it's more than fonts and font sizes, possibly non-dynamic mono-spacing rather than adaptive spacing, where each character is its own width.
If you "i" is the same width as your "m", then you're using monospacing—which is easier to see for older eyes, yet it does change how the text appears on the page.
However, you really can't change the font of specific lines to segments. You can, say if you want to include the dialogue spoken in a foreign tongue, but generally, you use a specific font rather than randomly switching them, as doing that makes it much more difficult cleaning it up afterwards.
Also, certain fonts are more distinctive than others. If your 't's have little tails, then you have "serif" fonts, if not you have "non-serif" fonts (which makes a difference when speed reading, as it makes it easier to distinguish patterns at a glance, essentially observing the word without actually reading it.
Still, your best strategy is to pick a specific font and then make it your standard default font and not switching them around without a specific reason.
As a graphic-designer, I've long designed my own covers as well as formatting each of my books (SOL only allows limited formatting changes, which is better for most authors).