admin-ajax.php file is causing slow page load

Dear Forum Members,

as I have been checking the speed performance of my wordpress site on https://www.kunstplaza.de with the Pingdom Website Speed Test, I have seen in the waterfall chart that there is one file of my wordpress installation that is slowing down the page load performance in particular:

https://www.kunstplaza.de/wp-admin/admin-ajax.php?0.7278787195682526

I have repeated the speed test several times and always got the same or very similar results. This file seems to be causing the web browser to wait for data from the server over 3 seconds (which is about 65-75% of the overall load time).

Does anybody of you have a clue what could cause this trouble and why this file is triggered when browsing the homepage of my website?

Thanks for your help! It is highly appreciated.