Delay before copying to remote directory

We had an issue recently for an upload job that normally takes 2 minutes but took 2.5 hours. Looking at the log of the task, we noticed instances where the file is listed and then a delay of 5-6 minutes before the copying starts, example below:
. 2017-03-01 22:41:50.740 File: '223810_686523.pdf' [2017-03-01T18:51:04.323Z] [5077109]
. 2017-03-01 22:47:18.012 Copying "223810_686523.pdf" to remote directory started.
................................
. 2017-03-01 22:47:20.027 Transfer done: '223810_686523.pdf' [5077109]

We are usually copying 300 or so files so that 5-6 minutes added up to 2.5 hours when it occurred multiple times. What is happening after file is listed, but before the copying starts? I am trying to narrow down the root cause of this huge time difference. The computer running the job did not have any unusual load or activity during the job. Any input is greatly appreciated.

Hardly anything is happening between those two log entries.

The machine and/or a disk must have been probably extremely busy for this to happen.

martin wrote:

Hardly anything is happening between those two log entries.

The machine and/or a disk must have been probably extremely busy for this to happen.

Do you think the remote side would come into play there? I checked the performance logs on my side during that time frame and there was no load on the machine. I will dig some more into the disk activity. Thanks!

Do you think the remote side would come into play there?

No, I do not think so. There's no communication with the server between those two log records.