in terms of configuration, in the opinion of friends, what is the best fit for « dpi », to get a
good resolution image and a reasonable rendering time?
I am doing some tests using dpi between 150 and 180, but the rendering time is very high, I believe
that this number directly affects the processing time, but I do not know how much is lost in
quality by lowering this number a lot.
in advance, thanks for the help 
Hello,
The rendering is always done in 72 dpi.
The dpi in the rendering settings dialog is just an indication to help you determine the rendering size in pixel width and height you need to set to obtain a printed image size at the set dpi.
As an example if I set the rendering size to 3840x2160 and the dpi to 72, I will get a printed image of 1.35 m x 0.76 m at 72dpi:
If I set the dpi to 300, I will get a printed image of 0.33 m x 0.18 m at 300dpi :
So changing the dpi does not change the size of the calculated image, it shouldn’t , therefore, change the rendering time.
It just changes the size of the printed image if you set the printer dpi to the indicated value.
Hope it’s clear 
