jEzEk, thanks for undertaking the tests. They show two things:
the RT values are as high as mine, up 100++ ms
in one test (the first) they went down to a normal value of 2-3ms
I think the problem in general is reverse DNS or ARP related. When I ping from my laptop my AP 192.168.178.1 I see constant values of 2-3 ms. When I ping some other laptop (an Ubuntu 18.04) I see this:
guru@c720-r342378 ~]$ ping 192.168.178.36
PING 192.168.178.36 (192.168.178.36): 56 data bytes
64 bytes from 192.168.178.36: icmp_seq=0 ttl=64 time=2.897 ms
64 bytes from 192.168.178.36: icmp_seq=1 ttl=64 time=2.993 ms
64 bytes from 192.168.178.36: icmp_seq=2 ttl=64 time=3.610 ms
64 bytes from 192.168.178.36: icmp_seq=3 ttl=64 time=2.852 ms
64 bytes from 192.168.178.36: icmp_seq=4 ttl=64 time=3.022 ms
64 bytes from 192.168.178.36: icmp_seq=5 ttl=64 time=3.024 ms
64 bytes from 192.168.178.36: icmp_seq=6 ttl=64 time=5.926 ms
64 bytes from 192.168.178.36: icmp_seq=7 ttl=64 time=3.230 ms
64 bytes from 192.168.178.36: icmp_seq=8 ttl=64 time=3.015 ms
64 bytes from 192.168.178.36: icmp_seq=9 ttl=64 time=343.706 ms
64 bytes from 192.168.178.36: icmp_seq=10 ttl=64 time=2.999 ms
64 bytes from 192.168.178.36: icmp_seq=11 ttl=64 time=3.006 ms
64 bytes from 192.168.178.36: icmp_seq=12 ttl=64 time=2.923 ms
64 bytes from 192.168.178.36: icmp_seq=13 ttl=64 time=5.357 ms
64 bytes from 192.168.178.36: icmp_seq=14 ttl=64 time=12.376 ms
64 bytes from 192.168.178.36: icmp_seq=15 ttl=64 time=2.988 ms
64 bytes from 192.168.178.36: icmp_seq=16 ttl=64 time=3.066 ms
64 bytes from 192.168.178.36: icmp_seq=17 ttl=64 time=3.007 ms
64 bytes from 192.168.178.36: icmp_seq=18 ttl=64 time=2.986 ms
64 bytes from 192.168.178.36: icmp_seq=19 ttl=64 time=317.896 ms
64 bytes from 192.168.178.36: icmp_seq=20 ttl=64 time=634.518 ms
i.e. only a few peeks up to 600 ms.
Now I went to the other laptop in question and inserted a line for my source IP 192.168.178.22 and for the laptop itself 192.168.178.36 in the /etc/hosts file as:
guru@m4400:~$ cat /etc/hosts
127.0.0.1 localhost
127.0.1.1 m4400
192.168.178.1 fritz
192.168.178.22 ubguru
192.168.178.36 ubuntu
and magically, the RT goes down to normal:
[guru@c720-r342378 ~]$ ping 192.168.178.36
PING 192.168.178.36 (192.168.178.36): 56 data bytes
64 bytes from 192.168.178.36: icmp_seq=0 ttl=64 time=3.010 ms
64 bytes from 192.168.178.36: icmp_seq=1 ttl=64 time=2.900 ms
64 bytes from 192.168.178.36: icmp_seq=2 ttl=64 time=3.124 ms
64 bytes from 192.168.178.36: icmp_seq=3 ttl=64 time=3.141 ms
64 bytes from 192.168.178.36: icmp_seq=4 ttl=64 time=3.422 ms
64 bytes from 192.168.178.36: icmp_seq=5 ttl=64 time=3.081 ms
64 bytes from 192.168.178.36: icmp_seq=6 ttl=64 time=2.919 ms
64 bytes from 192.168.178.36: icmp_seq=7 ttl=64 time=2.897 ms
64 bytes from 192.168.178.36: icmp_seq=8 ttl=64 time=3.005 ms
64 bytes from 192.168.178.36: icmp_seq=9 ttl=64 time=3.016 ms
64 bytes from 192.168.178.36: icmp_seq=10 ttl=64 time=2.888 ms
64 bytes from 192.168.178.36: icmp_seq=11 ttl=64 time=3.011 ms
64 bytes from 192.168.178.36: icmp_seq=12 ttl=64 time=2.923 ms
64 bytes from 192.168.178.36: icmp_seq=13 ttl=64 time=2.867 ms
To be investigated further ...