The really interesting part is the huge gap between loading the CSS and starting the load of "patternbg.png", which is linked to by the CSS. When I load https://lgms.nl/ locally, I see the CSS finish loading at the 397ms mark in the overall pageload and the image load get queues at the 411ms mark, which is more like what I expected.

My best guess is that there is some extension involved that takes a long time to process attempted fetches and decide whether to allow them or not.... It would be interesting if the timeline included "we are ready to go fetch this and waiting on extensions to make up their mind" bits.