Strange leaps for rendering time speed

[Belus92]

Hi there!
I have some strange leaps in speed rendering of image. When i push render button everythig go fine, my image with 2500x1850 resolution go with speed 1 pass per 19 second, but when 6 pass go for a rendering, render speed down from 1 pass per 19 second to 1 pass per 50 second, and i dont know what a reason for that downfall.
My computer configuration:
i7-4790
660 geforce
16 gb (my scene uses 9-10 gb when rendering)

The picture is here:

I will be happy if u would help me :)) Wish u best karma
PS: sorry for my bad english

[romullus]

That is expected. By default, after initial 5 passes Corona starts calculating adaptivity and time per pass may change significantly. There's nothing to worry about as by using adaptivity you get more evenly distributed noise, which usually leads to faster render times. However, if for some reason you don't like that behaviour, you can turn off adaptivity in devel/experiment stuff rollout.