Because the axes is getting "full" - not quite sure how else to describe it. Put the command "cla" before the call to imellipse and you'll see the times stay short no matter how many times you call it.
Some operations usinggpuArray don't complete synchronously, so usingtic andtoc can give you misleading timings. You should usegputimeit to timegpuArray operations to get reliable timings.
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