Broken pipe issue

Here is an issue I've found that I don't understand

When I set the number of items per page to 20 and read the comics from internet, everything is going fine: browsing is fast, and I don't see any problems

But, when I set the number of items to 24, then every time I load a new page some icons takes a long time to load (more thant 0sec), and I see 4 warnings in the logs

20161114 14:07:50 [pool-1-thread-20]  WARN com.ubooquity.e.e - Interrupted while sending response (perhaps the client stopped the download) - java.net.SocketException - Broken pipe
20161114 14:07:50 [pool-1-thread-13]  WARN com.ubooquity.e.e - Interrupted while sending response (perhaps theclient stopped the download) - java.net.SocketException - Broken pipe
20161114 14:07:50 [pool-1-thread-15]  WARN com.ubooquity.e.e - Interrupted while sending response (perhaps the client stopped the download) - java.net.SocketException - Broken pipe
20161114 14:39:59 [pool-1-thread-15]  WARN com.ubooquity.e.e - Interrupted while sending response (perhaps the client stopped the download) - java.net.SocketException - Broken pipe

I don't understand this issue nor do I know where to search for a cause; I don't believe it's a performance issue on my Synology NAS: a difference of 4 items on a page doesn't seem enough to create a problem, and I have a fast connection to internet, so I don't believe the problem is there either...

If I set the number of items to 26, I see that 6 items take a very long time to load... but I dont see any warning in the logs

If I set the number of items to 21, I see that 1 item take a very long time to load... but I don't see any warning in the logs either

What could be the cause of this ? why is 20 a magic number ?