Very slow on some pages when run from flatpak

I wonder why displaying some pages is very slow when Nyxt is run from flatpak. For example, it takes about 16 seconds to finish loading of Portál ČHMÚ : Předpovědi : Předpovědi počasí : Česká republika : Kraje : Jihomoravský with Nyxt from flatpak in my NixOS environment. When running Nyxt from a NixOS package, it takes less than 3 seconds in the same environment.
Does anybody know what could be the problem?

I haven't the slightest idea, but if you do find something, I would be very keen to know!

That is odd. Can you consistently reproduce it?

Can you consistently reproduce it?