Delay in ipv6

Hi all,

I hope this is not offtopic here. If so, pls let me know a better place 
for this question.

I configured wireguard to route both IPv4 and IPv6. Both protocols work 
fine. But test-ipv6.com only gives 9/10 points because the browser does 
not use IPV6 even though it is available.

Then I investigated a bit and found this:

ping -4 dns.google => ping sequence starts immediately
ping -6 dns.google => .5 secs delay => ping sequence starts
ping -6 dns.google => ping sequence starts immediately

i.e.: On the first try, ping6 takes longer, but the second time there is 
no delay anymore.

I suspected DNS trouble, but pinging 2001:4860:4860::8844 shows the 
exact same behavior: delay the first time, no delay next time.

Yet I couldn't determine how long I need to wait until a second try 
becomes a first one again, i.e. the delay shows up again.

Any ideas where to look next?

-- 
Andre Tann