by writing: "Firefox, safari and IE have constant behaviour in this" i mean that the other browsers takes always the same time to execute.

In the console getting the error as "GET https://www.deltalink.it/andreab/fabric/fabric.js net::ERR_INSECURE_RESPONSE" on Mac 10.11.6 using stable 53.0.2785.116.
Could you please elaborate more what has to be looked for further triage it.

I m very sorry, something went wrong when saving fiddle.
http://jsfiddle.net/emeagcnx/20/  fiddle updated.

Attach screenshot of console with filter timing:
As you see first time the functions take 115ms, then 600ms.
In other browsers does not happen.

Deleted: Screen Shot 2016-09-20 at 1.38.46 PM.png 153 KB

	Screen Shot 2016-09-20 at 1.38.46 PM.png 153 KB View Download

Investigating

I have some investigation.

I found that inside the fabric.js, there is a function applyTo around line 19000, and inside that function there is a giant for loop look like this:
for (var y = 0; y < sh; y++)
  for (var x = 0; x < sw; x++)
    ...
    for (var cy = 0; cy < side; cy++)
      for (var cx = 0; cx < side; cx++)
      ...

I put a timer surrounding this entire for loop block, and it takes about 150ms in the first time, and 270ms in the second time. I suspect there is some kind of v8 de-optimization.

Aside from that, I tried reverting this CL locally:
https://codereview.chromium.org/2330063002
and what I observe is that the entire for loop block takes 42ms in the first time, and 173ms in the second time, which is 100ms less everytime.
I am adding the author of the above CL, could you please take a look?

Oh, I forgot to mention, comment#6 is observed on Linux.

That big cycle is a convolution filter.
so while the 2 external loops are for rows and columns in the image and read the image as a buffer, the internal 2 loops are the convolution matrix and read data in the current line and also in the upper and lower lines of image.
i imagine this can confuse some caching logic.
Firefox was once affected but they solved one day. I do not know in wich revision, maybe is understandable by the dates in the issue tracker of fabricjs.

any news for this matter? some code path are still heavily slowed down after first run

http://jsfiddle.net/emeagcnx/30/  i updated the fiddle since the linked lib ahd an api change that broke it.
The situation improved a bit, i m unsure if is chrome 59 and the underling changes.
First execution ~95ms, after ~170ms. This is a 2x slowdown vs a 4x-5x slowdown we had before.