This page allows you to define how carefully the graphics are drawn in the plot display area. By default any text is drawn using anti-aliasing, which makes the graphics less jagged, and spectra are drawn without anti-aliasing. If you'd also like the spectra to be anti-aliased tick Everything:.
It's not possible to anti-alias the spectra and not the text (so it's not just a perverse choice of options). Also note that any rendering options are applied to any captured JPEGs produced in the plot.
Finally anti-aliasing is a CPU expensive procedure so it can result in a significant performance degradation (so the default is switched off for spectra).