does top show the cpu going to 100%?

(In reply to Albert Astals Cid from comment #1)
> does top show the cpu going to 100%?

No, my CPU acts normally.

Sometimes similar behaviour happens to me when the disk is busy (I usually use atop to find out)...

Is your disk busy as Jonathan suggests?

No. When it freezes, I don't see any high disk I/O activity. I checked it with iostat while changing pages.

Can you record a video and upload it somewhere so we see what happens? Also try resizing the window while the freeze is happening.

(In reply to Albert Astals Cid from comment #6)
> Can you record a video and upload it somewhere so we see what happens? Also
> try resizing the window while the freeze is happening.

In the next link you have two videos with statistics provided by the console (with atop using disk activity option): the first one using only my keyboard to pass every page, and the second one using the touch-pad. I worked with two random eBooks. For the video with the keyboard, in some cases I maintained pressed the 'down' key all the time. Also I kept pressing that button repetitively (to clarify that when you see so fast intermittent pauses while the page is going down is not because it is freezing at those moments but I was pull down the down key and immediately I released it).  If you see high CPU consumption in the shell, it's because the video recording and not for the process of scrolling pages.

https://spideroak.com/browse/share/hikari/Okular/VideoOkular/

Another observation: in some cases when I use the "Contents" menu and select a chapter or something like, Okular freezes too a piece of seconds.

Just to make sure plase, attach one of the documents you're using

(In reply to Albert Astals Cid from comment #9)
> Just to make sure plase, attach one of the documents you're using

Ok. I added a file in https://spideroak.com/browse/share/hikari/Okular/VideoOkular/. Try to change among pages 502 and 503, (section 18.2) for example. Also, try to go to different sections in "Contents" menu.

can't reproduce any kind of slowlyness, everything is silky smooth, can you removing (save them before in case you want to restore them) the files at .kde/share/config/okular* and see if it makes any difference?

(In reply to Albert Astals Cid from comment #11)
> can't reproduce any kind of slowlyness, everything is silky smooth, can you
> removing (save them before in case you want to restore them) the files at
> .kde/share/config/okular* and see if it makes any difference?

Sorry, but no changes.

This is unfortunately as far as i can go since i can't reproduce it.

If you're technically minded i suggest seeing if strace shows something weird or if by using gdb you can see where the process is stuck.

If you find something interesting there we may have something to work on, otherwise with no way to reproduce it ourselves it's almost impossible to fix.

I found a strange behavior. When I deactivate my internet connection, Okular works smoothly, there is not freezing.

That's *very* strange

strace or ltrace in both situations(with and without internet) may show something different and help debug the issue.

(In reply to Albert Astals Cid from comment #15)
> That's *very* strange
> 
> strace or ltrace in both situations(with and without internet) may show
> something different and help debug the issue.

There are two new files in https://spideroak.com/browse/share/hikari/Okular/VideoOkular/.
One of them is Okular working with Internet and the other without it. While I was scrolling pages, I could identify a similar behavior when the page were changed completely. Maybe these lines help (I paused the console where a page changed to catch them, it not is accurate):

   Without Internet: line 65194
   With Internet: line 50522

Something is making a dns request (186.177.67.145" is your dns server i gather) for "compu".

Seems to me your cups configuration or something else is trying to reach something you have configured as "compu" and is failing.

(In reply to Albert Astals Cid from comment #17)
> Something is making a dns request (186.177.67.145" is your dns server i
> gather) for "compu".
> 
> Seems to me your cups configuration or something else is trying to reach
> something you have configured as "compu" and is failing.

Do you need any additional information?

I can't do more than hint at what may be happening, okular is not the one directly doing that web request, something in your system is, probably cups, but that's just a random guess, i'd suggest you to use gdb to start okular and be fast enough so that while okular is frozen you go to gdb, Ctrl+C and take a backtrace of what's going on.

Do you know how to use gdb?

(In reply to Albert Astals Cid from comment #19)
> I can't do more than hint at what may be happening, okular is not the one
> directly doing that web request, something in your system is, probably cups,
> but that's just a random guess, i'd suggest you to use gdb to start okular
> and be fast enough so that while okular is frozen you go to gdb, Ctrl+C and
> take a backtrace of what's going on.
> 
> Do you know how to use gdb?

I'm sorry but I don't know how to perform debugging  with it. I will looking up but if you have some specific steps to give you better information, would be the best.

do this in a terminal
gdb okular
run
   Open the file in okular and make it get "frozen"
   Quickly change to the terminal where you are running gdb before it unfreezes
   Press Ctrl+C
backtrace

And attach the output

I removed my printer (I had only one) from "Printers" menu in "System Settings" and Okular works perfectly. No more delays.

I told you ages ago to investigate if the printers where involved, thanks for ignoring me for so long