I have never come and cross such a thing like that i know that all selector modes are use to return a search which return the fields and events depending on the selector mode chooses and from there you can save it as dashboard panal base on the events and fields return by the selector mode use.
or do you want to Set search mode to adjust your search experience if so i can help you read on Splunk-6.1.1-Search this will help you faster to know what this mode are use for.
i have never see a dashboard been save as panal is been display by a selector mode .
Note that every search that is been make by using the selector modes will return the number of events and fields depend on the mode selected and which it is save as dashboard panal depending on the number of fields and events the selected modes return .
Note That the dashboard you save as panal it will display fields and events at the process in which it is save as dashboard panal not in the selector mode again
The only way I can help you for at this moment I will like you to check the hardware material of the machine that you are using because most problem that I have come and pass through went I want to verify on the material of the hardware machine it is not what splunk has Recommended due to this it cause dashboard not render quickly .
I will like you to check on the Recommended and minimum hardware capacity in the documentation
Splunk-6.1.1-Installation if the problem can be material of hardware
Another problem maybe the search request that you have make, you know that there are search request that are more consisting or heavy than the search head ,so during the time that you want to display the dashboard it can take a lot time to display it due to the search request that is more consisting or heavy this is what I can help you at this moment