it's not yet possible to set the bandwidth limit for backup jobs in the storage configuration, it can be done on a per node basis by setting the option in /etc/vzdump.conf or on a per job basis by using the CLI option (see man vzdump), and editing /etc/pve/vzdump.cron (best to try running the modified command before editing the file). Here is the feature request for this.
So if that is not the problem we are in the dark, maybe you can describe your setup in more detail, e.g. network, storage and hardware. Also if you have access via the shell check the syslog, dmesg ecc. for errors/infos and ps/top/atop/htop/iostat or whatever you use to see system load ecc.