The more results you try to load per page the larger the data set is that has to be passed from the API.
Obviously, the more data you try to pull the longer it will take, however your hosting server and PHP.ini settings can effect both load time and potentially block larger API calls entirely.
If you need to load a lot of results per page you'll need to check your PHP.ini settings and make sure your max_input_vars are set to at least 3,000. This setting limits the amount of data that is allowed to be passed over the URI.
If you're not sure where or how to adjust your PHP.ini settings there are a couple of free plugins that you can install for Wordpress that will allow you to change it from the dashboard.
Firewalls or internal hosting server settings could also block large data requests. If this is the case you'll need to speak with your hosting company to see if this can be changed.
Another factor beyond just pulling the data is actually processing it. Once the data is pulled it's then processed via PHP in order to be displayed in your browser. Several factors including your hosting server's speed, version of PHP installed on your server and how your PHP is setup can all effect how quickly that data is processed and then displayed.
An inexpensive shared hosting account will likely take a lot longer to process the data than a more robust VPS hosting server with more CPU and RAM.
PHP 7.X is about 3 times faster than PHP 5.X and PHP cache options such as memcache, OPcache, APC and Xcache can also effect PHP performance. You'll want to make sure you're on an up-to-date PHP version, you have a good PHP cache option installed and enabled and you'll want to ensure your PHP.ini settings are optimized to ensure that not only the form results load as quickly as possible, but everything on your website loads as quickly as possible.
While adjusting your max_input_vars is relatively quick and easy, server-side performance optimization is complex and should only be done by someone that really knows what they're doing. At the end of the day, if loading 250 results per page takes too long or you have issues with the results loading, it's probably easier to just load 100 results per page.
One final thing you can try if you're able to load 100 results per page but not 250 is to just load 100 at a time and hit the next button 3 times to cycle through the results. If the results load all 3 times then it's definitely a server-side issue with handling the larger amount of data at once. However IF results fail to load when you hit next then there could be an issue with one of your form's data causing it to break. If this happens, please make a support ticket so that we can help resolve.