Approval Inbox Perfomance
It all depends on the number of records and operato'sr approval rights.
If did not customized the extra field on inbox then you cannot do anything.
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For the default view displayed when you access the Approval queue, check to see if multiple grouping/sorts have been configured in the view definition. Excessive grouping/sorting can increase response time on views.
Please check the following Document: http://support.openview.hp.com/selfsolve/document/KM961884
It may related to your problem about Approval Inbox. It described:
The description is made for the example of table "Approval" in Service Manager (SM) 7.11 OOB system.
If you want to use the field "current.pending.groups" in your inbox queries, the performance is very bad.
Please check. Hope this helps!