guix pull: computing Guix derivation takes forever

[akib via]

I've just installed Guix on a partition of my new HDD.  After the 
installation I logged in to my user account on a Linux console and 
executed 'guix pull'.  After that it pulled the repository and computed 
Guix derivation, but stuck while updating substitutes.  So I thought 
something is wrong and restarted the command.  Now the pull operation 
always stucks while computing Guix derivation.  There is no sign of 
activity according to top.