Steps:

I started pacman upgrade.
During the download stage I put my system to sleep during "gradle" package download.
The next day I continued the upgrade. The time that passed was around 19h.
Download speeds are reported to be negative :) And the output is repetitive.
PS. I used pacman indirectly via yaourt, but believe it to be a pacman bug.

Here is my console output:
[...]
python-sphinx-1.6.4-1-any 1357,8 KiB 1435K/s 00:01 [####################################################################] 100%

error: failed retrieving file 'gradle-4.2.1-1-any.pkg.tar.xz' from mirror.bytemark.co.uk : Operation too slow. Less than 1 bytes/sec transferred the last 10 seconds--------------------] 60%
error: failed retrieving file 'gradle-4.2.1-1-any.pkg.tar.xz' from mirror.cinosure.com : Could not resolve host: mirror.cinosure.com
gradle-4.2.1-1-any 1147,0 B -7,42M/s --:-- [--------------------------------------------------------------------] 0
gradle-4.2.1-1-any 16,7 KiB -4,94M/s --:-- [--------------------------------------------------------------------] 0
gradle-4.2.1-1-any 42,1 KiB -3,27M/s --:-- [--------------------------------------------------------------------] 0
gradle-4.2.1-1-any 98,7 KiB -2,13M/s --:-- [--------------------------------------------------------------------] 0
gradle-4.2.1-1-any 158,1 KiB -1395,96K/s --:-- [--------------------------------------------------------------------]
gradle-4.2.1-1-any 600,7 KiB -192,97K/s --:-- [#-------------------------------------------------------------------]
gradle-4.2.1-1-any 25,9 MiB 209K/s 02:07 [####################################################################] 100%
[...]