I assume the performance impact would be minimal. From a quick look, it would only affect schema instantiation.

The benefit is that when using Python 3.7+, all schemas would be ordered. They still wouldn't return OrderedDict instances unless ordered Meta is passed, but they would keep fields declaration order in their output and in their schema list, so that order would be respected in API responses and apispec users would get their fields documented in declaration order.

Currently, users can achieve this in a base schema with

class OrderedBaseSchema(Schema):
    set_class = OrderedSet

My gut feeling is that this change would have no functional downside and a negligible perf impact at field init (less that we care about when adding a new feature), while using ordered=True does have an impact on serialization performance (I didn't measure it) and functionality (equality test depends on order), hence my proposal. There would still be an interest in allowing the use of OrderedDict.

If we go this route, we could use OrderedSet by default in marshmallow 3.x, and in marshmallow 4 perhaps rework the ordered feature because schemas would already be ordered so it would be specifically for users really needing an OrderedDict instance. Perhaps drop the ordered Meta option and let the user specify a dict class of their choice another way. But that's another story.

See marshmallow-code/flask-smorest#228 (comment).