Just running this end point with the default options takes 6 seconds on my forum:
https://invisioncommunity.com/developers/rest-api?endpoint=downloads/files/GETindex

This behaves the same with a C++ desktop application, or with a PHP script running on the same server.

A little debugging yields this information:

time_namelookup: 0.144395s
time_connect: 0.238170s
time_appconnect: 0.449827s
time_pretransfer: 0.449919s
time_redirect: 0.000000s
time_starttransfer: 5.473372s
----------
time_total: 6.230231s

Is this typical? Any ideas why this might be happening? I have not upgraded to version 5 yet.

Edited March 17Mar 17 by Interferon

It seems highly dependent on the number of results returned. Just using an API key and a browser, when perPage is set to 1 I estimate it takes about one second to return. With perPage set to 100 it takes about 23 seconds.

But when I am browsing through the web interface, the page load times are very fast. Each page of 25 results loads in about a second or less.

Edited March 18Mar 18 by Interferon

It will very much depend on your server. However I would say, you are quite a few versions behind there. So the first thing you need to do is update your site to the latest release

The upgrade from 4.2 to 4.7 seems to have fixed this. API responses seem to be about the same speed as the rest of the site now.