Info provided by Wouter Huitzing (prior to his 2019 FKT run):
The Pieterpad is the most famous and longest* hiking trail in The Netherlands, spanning an official distance of 498 km. It starts in the north in Pieterburen, and goes to the south (St. Pietersberg), where a continuation on the European trail GR5 is possible. Of course the reversed direction is possible too. The trail is mostly hiked by older people, walking one or two days during the weekend, but a few fast attempts have been made.
The fastest time I could find is set by Lucja Leonard and Marina Ranger, starting 27th July and finishing in 5 days (exact time unknown). Their progress was documention on the Facebook-page Simply Runderful and both wrote detailed blog posts afterwards (Marina, Lucja). This was fully supported. [Faster times are listed below.]
The official Pieterpad website has a news item (Dutch) stating Edwin van Nieff completed the trail in six days (16th - 21st May 2004). This probably makes him the fastest male, but I have no idea if this is supported or not, not much details can be found about this performance.
The trail also had a number of stage race attempts, called the PieterROGPad (Dutch), having the finishers in 7 days. Fastest net time was set by Karl Graff in 2000: 55 hours and 3 minutes. Fastest female was Ria Buiten in 2001: 60 hours and 23 minutes. Of course the net time is not really a reliable metric for things like this, it would be faster to just run 1 km a day for two years.
Not many other attempts are documented. There is not much of a FKT or thru-hike spirit in The Netherlands, so I don't think many attempts have been made.
* The Kustpad (Coastal Path) is longer, with a total of 721 km, but this one has been split into 5 parts, I can't find any official documentation if this is 1 trail or as 5 trails.