You are subject to [throttling](https://api.stackexchange.com/docs/throttle).

You can get more calls by  first [registering](http://stackapps.com/apps/oauth/register) your application. After that you get access to a detail page which includes a value for `key`.  The description for that field is:

> Pass this as key when making requests against the Stack Exchange API to receive a higher request quota.

> This is not considered a secret, and may be safely embed in client side code or distributed binaries. 

Once you have your key, the url to use becomes:

`/users/+J+/questions?order=desc&sort=activity&site=stackoverflow&key=*somekey*`

I have an example [here](http://meta.stackexchange.com/a/269020/158100) where I use a `key` query parameter in a similar way.

Keep in mind to still check for the existence of a value in the `backoff` field in the [wrapper response](https://api.stackexchange.com/docs/wrapper). It indicates the number of seconds to wait before you are allowed to make the next call.