I've tested this, and it does seem that adding additional predicates to the CompositePredicateDescription takes an increasingly long time once you've gone past a certain number of predicates. I'm not sure if this is a limitation of DevForce or EntityFramework, so I'll open a defect ticket for us to investigate it.

Tyring to OR more than 10-12 single predicate descriptions for the same property one a time into a composite predicate description causes timeouts on the entity server and each time you OR a new predicate, it gets progressively slower, getting really bad as you get over 15 ORs.  The last call we see (before hitting the timeout) is to IdeaBlade.Linq.CompositePredicateDescription.InstanceType.get() line 85.

The property in this case was an INT field (but this probably happens for any type) and each predicate was just using the filter for equality.  The code that creates these is very dynamic but essentially, after first creating a single predicate description, if it finds another predicate for the column, it builds a new predicate description and combines the first 2 single predicates into a composite.  Subsequent predicates on that same property uses the composite to combine it with an OR on each new predicate. 

// test for > 20 predicates ORed together                         PredicateDescription p1 = null;                         CompositePredicateDescription c1 = null;                         for (int i = 0; i <= 20; i++)                         {                             var p = new PredicateDescription(typeof (Product), "SupplierID", FilterOperator.IsEqualTo, i);                             switch (i)                             {                                 case 0:                                     p1 = p;                                     break;                                 case 1:                                     c1 = p1.Or(p);                                     p1 = null;                                     break;                                 default:                                     c1 = c1.Or(p);                                     break;                             }                         }