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Because I have completely opposite result during this render. Cube was subdivided 10 times in edit mode + 3x subsurf modifier = more than 40k faces. And 582 faces after Decimate No decimate Decimate

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  • $\begingroup$ I made similar experiment and render times match the expectations. The shortest time with default cube. The longest with Subsurface, shorter with subsurface and decimate.Almost back to the shortest after applying both modifiers(so the last render is of cube with 16 faces). $\endgroup$ – Mzidare Nov 18 '16 at 11:48
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    $\begingroup$ Decimate modifier is single-threaded and slow. On high-poly meshes it takes ages to compute (sometimes more than it takes to render). Modifiers are computed at pre-render time in the phase "Synchronising object". Apply the modifier. There is only a small vertex count range where the modifier will actually speed up anything non-applied. $\endgroup$ – Jaroslav Jerryno Novotny Nov 18 '16 at 13:09
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Principally, the decimate modifier reduces face count. A smaller geometry leads to less memory usage and a shorter render time.

BUT: The modifier is slow. Really slow (Don't blame it, reducing geometry isn't a simple task). Before rendering, all modifiers are "applied" so that they can get rendered in the "Syncronizing object" phase as Jerryno said. This will add some overhead to your rendering time.

If you can afford it, apply the modifier. Then it will calculate once and for all instead of at every render. Eventually back up your object somewhere else just to be sure.

Also please keep in mind that the decimate modifier is a "I messed up the polygon count" quick fix. If you need it, in most of the cases that's a hint that you made a mistake somewhere. See here for alternative solutions.

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  • $\begingroup$ Thank you, I get it. It was a quick test, but my main project is animation with transforming, so applying is not for me. So I will stick with rendering to EXR files and then "compose only" process. $\endgroup$ – Serge L Nov 18 '16 at 18:54

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