Plotting Strangeness?

[oldhpfan]

I was playing around plotting equations in the advanced graphing app to get fun shapes.
These two equations, and variants, produce some weird behavior when plotting, it's like the calculator (and emulator, though faster) are plotting a bunch of extra points then eliminating them to get the correct line in parts of the plot.
X=Y^3 -5*Y^2 + 15
X=Y^4 -5*Y^2 + 15 (looks like a sigma)
Really slows down the plot. Any ideas what is going on? I have not noticed this yet when plotting functions and other non functions like X^2+y^2 are fine.