When searching for an non-indexed field object in Salesforce, some user might encounter a "request time out" error from Salesforce. This could be due to the system taking too long to find the requested object.
In Salesforce, some fields are indexed and are "given higher priority" during a search function and hence a faster search output. When a custom object is created and not indexed, the time out issue might occur.
To improve performance of the formula with custom indexes and how to create custom indexes, view the link here: https://help.salesforce.com/apex/HTViewSolution?id=000006007&language=en_US