Is GUID(uniqueidentifier) not suitable for a clustered index?
It will slow down inserts rather than queries. That's because it is essentially inserting a record somewhere randomly inside the existing records, instead of just whacking it on the end (as it would if you were using an identity or sequential dates).

You can somewhat reduce this by using NEWSEQUENTIALID instead of NEWID (assuming the GUID is generated by the database rather than your application).