Extracting values from dictionaries where the keys match
If I'm understanding it correctly, you're populating a ConcurrentDictionary from the values of two other ConcurrentDictionaries, where the keys are equal. 

Try this, it's vastly faster than your loop in my tests.

var matches = FirstDictionary.Keys.Intersect(SecondDictionary.Keys);
foreach (var m in matches)
    ResultDictionary.TryAdd(FirstDictionary[m], SecondDictionary[m]);