In an extremely simple test of loading a 140 MB JSON file, ujson ends up with 30% higher memory usage even after loading has happened.

Both built-in json and simplejson uses exactly the same amount of memory: 992 MB
ujson on the other hand uses: 1314 MB

Test is super simple, like:

import json
import ujson
import time

with open('test.json') as infile:
    data = ujson.load(infile)

time.sleep(60)

And I check in htop's RES column for the memory usage. It's the same as macOS's Activity Monitor's memory column.