time query to google always more than 2 seconds

I have a strange problem with my computer. The configuration is

Operating System: Ubuntu 14.04.4 LTS
Keel: Linux 3.19.0-43-generic
Architecture: x86_64

Irrespective of if I use eth0 or wlan0, the query to google.de with the command

time dig www.google.de
; <<>> DiG 9.9.5-3ubuntu0.8-Ubuntu <<>> www.google.de
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 34093
etc etc ..... ( response edited )
;; Query time: 17 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; MSG SIZE  rcvd: 58
real    0m2.035s
user    0m0.008s
sys 0m0.008s

sometimes ( not always ) takes more than 2 seconds.

However, sometimes it also finishes in some milliseconds. I have observed, that if the network is slow or fast is decided at the time when the connection is established. If the connection is established and the query to google is fast, then it remains like this too till I try to connect to another access point. If during the establishment of the network connection, the query is slow, then it also remains like this throughout the session.

I tried

rfkill block wifi
rfkill unblock wifi

dhclient -r
dhclient wlan0

ifdown eth0
ifup eth0

sudo tee  /proc/sys/net/ipv6/conf/all/disable_ipv6 <<<"1"

For me it looks like that the query to google is done in 17 msec, and hence the network adaptor works fine, but the OS waits for something and at a timeout of 2 seconds, delivers the result to the console. Just dont understand why? Any hint would be very helpful.

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