Created attachment 12906 [details]
The sample file.

Not cairo specific, actually cairo seems faster than splash

1,1 seconds with cairo backend, I think it's reasonable, moving to splash. 

I had a look into that sample: It seems, as if cairo produced a lot a transparency group entries in the PDF, which slows down rendering in splash. According to a the PDF spec (11.4.4 Group Compositing Computations, page 337, note 5):

As a result of these corrections, the effect of compositing objects as a group is the same as that of compositing them separately (without grouping) if the following conditions hold:
1. The group is non-isolated and has the same knockout attribute as its parent group (see 11.4.5, "Isolated Groups," and “Knockout Groups”).
2. When compositing the group’s results with the group backdrop, the Normal blend mode is used, and the shape and opacity inputs are always 1.0.

it should be quite easy to recognize this in the splash transparency group functions and "remove" this behaviour in splash. I already tested it, hard coding transpGroup to gFalse in Gfx: rendering time less than one second, result seems to be the same.

I'll try to implement it for 0.22.0

Created attachment 60877 [details] [review]
remove unnecesary transparency group handling in splash

This patch avoids unnecesary transparency group handling in splash and renders the sample PDF really fast.

Awesome job
Pushed :-)