Users in UMP (either account contact or administrator users) who have access to a large number of groups, especially Interface groups, may experience very long load times in USM and slow performance.
A hotfix is available which corrects an issue with loading interface-based groups. It can be downloaded from the UIM Hotfix Index Page.
The hotfix to apply is ump_usm_8.4.7_HF1 (or higher, depending on what is available on the above page.)