Currently, it is not possible to easily determine the list of Collections that a user has SUBMIT privileges to. We are therefore having to essentially "crawl" the Community/Collection hierarchy looking Collection by Collection for ones the current user can submit to. This results in performance issues in filling out a dropdown of Collections in the Angular UI as noted in https://github.com/DSpace/dspace-angular/issues/487
As discussed in today's DSpace 7 meeting and in the proposal at https://wiki.lyrasis.org/display/DSPACE/Collection+dropdown+in+submission, we should implement a lookup via Solr to solve this issue.
The solution will be to add a plugin, very similar to the current SolrServiceResourceRestrictionPlugin, that indexes which groups can submit in a given Collection within the Solr record of each Collection. This will provide improved performance in looking up Collection submit permissions, as you can search Solr for all Collections that allow one of your Groups to submit.
The initial estimate for this work is 1 day (8 hours).