If you have recently upgraded to Sitecore 9 and you see that the publishing is getting queued frequently then this is the post you should look for.
Publishing queued in Sitecore 9
We saw the publishing getting queued frequently after upgrading our Sitecore to version 9. However the queue is getting cleared after a long time and items getting published normally. After going through the logs and creating a ticket with Sitecore, we got to know that in Sitecore 9 there are additional items getting added to the publishing list then we check the Publish related Items options.
As most of the content authors work from Experience Editor and do publish with related Items checked, there are more number of items added recursively to the publishing queue by Sitecore 9. As a result of this, Sitecore is processing more items and adding more items to the publishing queue. This is causing the delay of each publish and any concurrent publish is getting queued.
To make the publishing in SItecore 9 work as the previous version, we need to set the DeepScan option to false in the "Website\App_Config\Sitecore.config" as shown below. This would make the Sitecore 9 publishing (with related items) work like the Sitecore 8 publishing.
<processor type="Sitecore.Publishing.Pipelines.GetItemReferences.AddItemLinkReferences, Sitecore.Kernel">
After updating this setting to false we never saw the issue of publishing getting queued.
Hope this helps you. Please do comment your thoughts.