From over 10 years ago (I’m sorting through my old bookmarks). A single object looks like this:

{
    "ACCTOUNT_NUMBER":"1234567890",
    "CUSTOMER_NAME":"ACME Products and Services, Inc.",
    "ADDRESS":"123 Main Street",
    "CITY":"Albuquerque",
    "STATE":"NM",
    "ZIP":"87101-1234"
}

He tested the usability of a given browser while it loaded between 1 and 1,000,000 such records.

From this test, I am considering the sweet spot to be around 10,000 records at (1.55MB). The maximum number of usable records I would push to a browser would be around 25,000 records (3.87MB). Keep in mind there are numerous factors to keep in mind when determining how many records you should return to your JavaScript application. The purpose of this test was to help identify a general maximum number for conversations around large record sets with JSON.

Would love to see an updated version of the tests.