Admin.ajax causing delay in load time

Hi,

When I test my website (slaapwijzer.net) with tools.pingdom.com I see admin.ajax is causing a significant delay in load time (1,5 seconds on average). It only happens on blog posts (for example slaapwijzer.net/slecht-slapen) not on pages.

Does anybody know how I can fix this? I tried everything that is recommended but nothing works:

– I installed the Control Heartbeat plugin
– I added a code to functions.php
– I disabled all plugins
– I disabled the theme
– I deleted all post revisions etc.

Any ideas? Thanks in advance!