When FileMaker needs to fetch the value of a global variable, it needs to look it up internally I imagine. This goes incredibly fast, so I imagine the question to be a no-brainer.
On second thoughts, I imagine that if the list of global variables is beyond a certain length it might be faster to find a record in a small table. Is this the case, and if so, when would finding the record be faster? I'm thinking of finding a record through an ExecuteSQL function.