Hi Clemens,
Thank you for the question! Just to clarify, when you say "import" I'm assuming you mean making requests to get data using the Search method on the Google Ads API, is that right?
It's expected that these queries to the Google Ads API would be slower than similar queries made to the AdWords API. It's not just requesting resources, we consider the entire Search method to be under performing. In other words it's not that some queries are slower than others, the entire endpoint is slow.
Our primary focus right now is to improve the performance of Google Ads until it's at parity with AdWords. However there isn't a specific threshold for how fast the Google Ads API will be once it exits beta.
Thanks!
Ben Karl, Google Ads API Team