Christopher Brian Colohan writes:

Drag. In a slip you are sliding sideways through the air. This means
you are trying to push the side of the fuselage through the air. It
is not designed to be very aerodynamic going sideways, so when you do
this you will create all sorts of extra drag which the plane does not
normally have.

That explains it. Thanks. Slips may come in handy for me since I
never seem to be able to roll out to final with any reasonable
alignment with the runway. I suppose in real life I could look out a
side window and estimate my alignment better, but somehow I think it
might still be difficult for me. The other legs work out okay; I'm
flying nice rounded rectangles now (MSFS lets you analyze your ground
track and altitude profiles, so you can easily check your work).

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