[gentoo-user] Slow local network - how to debug?

[Helmut Jarausch]

Hi,

I have no experiences in debugging a slow network.

At home, I have a router which is connected to my telephone line via  
VDSL2.
I have 2 PCs one of which is connected by an ethernet cable (i.e.  
wired) while the other
one uses a wireless connection which is specified as 56 Mbit/s.

When  copying data from one machine to the other one I see varying  
speeds from only 0.5 Mbits/s up to
more than 20 Mbits/s. I have no idea why it is so slow some times.
There are no other wireless devices nearby.

What tools and techniques can I use to debug this situation?

Many thanks for a hint,
Helmut.