Could there be a reason why System.Type.ForName would take a lot of time to execute?

Salesforce has to "compile" the class, so if it's not in the code cache, it can take significant time for system type. This should not happen constantly, except after deployments or maintenance periods. If you're getting reconcilable poor performance, consider opening a case with Technical Support. However, it should not usually take more than a fraction of a second to assemble the code under normal circumstances, although it does depend on the complexity of the class (e.g. how many other classes are referenced, etc).