The TemplateMeta metaclass in utils/wrappers.py replaces the __instancecheck__ and __subclasscheck__ methods so that it can look up the type being checked against its registry of templated classes. The registry check is written incorrectly, though, checking the type against the registry keys instead of the registry values. The result is that it falls through to an expensive loop checking isinstance for each type in the values.

The fix should be a simple addition of .values() to the if statement.