zypper uses multilink download and probably aria2 client to do it.
Is correct when using download without proxy as it shoud enhance speed because
of the conteporary download of "slices" from vary (5) sites at a time.
When it's used a proxy, (e.g. squid) this slow down the download and misuse the
band usage for the client and even for the mirrors because of the fact that
squid don't allow to download only a slice of file but it try always to
download all the file, so when zypper is used with proxy every file is started
download 5 times so the band usage is something like 5 times the band usage
without proxy

I think the easy way is to put

download.max_concurrent_connections = 1

in /etc/zypp/zypp.conf when proxy is enabled

(also when pressing F4 in install dialog and setting the proxy)