Consider this example(taken from Thomas's question here)
rv = RandomVariate[ExponentialDistribution[2], 10^6];
t = AbsoluteTime[]; ListLinePlot[rv, PlotRange -> All]
AbsoluteTime[] - t (*Put this line into the NEXT cell,and evaluate \
both cells together*)
t = AbsoluteTime[];
Style[ListLinePlot[rv, PlotRange -> All], Antialiasing -> False]
AbsoluteTime[] - t (*This line in separate cell!*)
and this example (taken from a related question here)
ls = Table[Sinc[x*y], {x, -10, 10, 0.1}, {y, -10, 10, 0.1}];
ListPlot3D[ls, PlotRange -> All,
InterpolationOrder -> 2] // AbsoluteTiming
I get different performance in version 8 and version 9. Version 8 seems to be about twice as fast as version 9. Why this happens and how to make version 9 as fast as version 8?

AbsoluteTiming[ToBoxes@ListLinePlot[rv, PlotRange -> All];]in both 8 and 9. So it's likely not because of system APIs, @Liam, but because of Mathematica itself. – Szabolcs Jun 05 '13 at 12:08