When on a Change Request, there is a Related list called 'Changed CIs' which can have relational Configuration Item records added. When adding, an "Add All" UI Action is available to add all CIs that have returned in the filtered list.
This can cause extensive loading times if that list of CIs is immense. If you are only adding a few CIs but still experiencing extensive loading times (over a minute for a few CIs), please see below.
Release or Environment
Any custom Business rules on relational/task CI tables (such as task_ci) may cause this issue. If the Business Rule is checked to run on Update, when a CI with additional relationships is added, this Business Rule ( and potentially any Script Include it triggers) may run, for every relationship.
Deactivate any custom Business Rules on task_ci *or any other ci task table that query on update* to test load times. (test on Sub-production instances first)