Performance impact of Nested with many vs List of Nested.

[mspiller]

We noticed that Nested is slower than List of Nested. Not sure why this happens but should they be the same?

import timeit

import marshmallow
from marshmallow import fields
from marshmallow import validate


class PropertySchema(marshmallow.Schema):
    name = fields.String(validate=[validate.Regexp('^[0-9a-zA-Z -]{1,100}$')], required=True)
    value = fields.String(validate=[validate.Regexp('^[ -~]{0,255}$')], required=True)


class TrackInteractionActionSchema(marshmallow.Schema):
    properties = fields.Nested(PropertySchema, many=True, required=False)

class TrackInteractionActionSchemaNew(marshmallow.Schema):
    properties = fields.List(fields.Nested(PropertySchema), required=False)


def load_schema(schema: marshmallow.Schema, data: dict) -> None:
    try:
        schema.load(data)
    except validate.ValidationError as err:
        pass


if __name__ == '__main__':
    data = {
        "properties": [
            {"name": "x!0", "value": "v"} for x in range(20000)
        ]
    }

    a = TrackInteractionActionSchema()
    print(timeit.timeit(lambda: load_schema(a, data), number=10))
    b = TrackInteractionActionSchemaNew()
    print(timeit.timeit(lambda: load_schema(b, data), number=10))

It is couple of times slower:

7.725316457916051
1.1829904590267688