This one puzzled me for weeks, but I've finally figured it out.
For some reason the container image has a different set of default settings than the installation script that you get from nzbget.net. In the download queue settings make sure that you have a setting for ArticleCache (default is 100 MB) as well as WriteBuffer (default is 1024 kb). My container had blank entries for both of these. Before settings changes my downloads were limited to about 10MB/sec. After adding these entries downloads accelerated to 60MB/sec. I hope that helps.
I'm running Docker in a VM (4 vCPU / 4GB RAM) managed by Portainer.
Joe