If for some reason you need to reduce the download speed of emerge when downloading sources you can use a tweak in portage’s make.conf as explained in the handbook.
To keep wget and just add the bandwidth limit, add this to /etc/portage/make.conf:
Of course, adjust your rate to your need.
Update 2020: This method may certainly not work anymore but I don’t have a Guix installation to try.
I’m new to Guix, it’s a wonderful system but it’s such different than any other usual linux distribution that it’s hard to achieve some basics tasks. As Guix is 100% free/libre software, Firefox has been removed and replaced by icecat. This is nearly the same software but some “features” has been removed (like webRTC) for some reasons (security, freedom). I don’t blame Guix team for that, I understand the choice.
But my problem is that I need Firefox. I finally achieve to get it working from the official binary downloaded from mozilla website.
You need to install some packages to get the libraries, which will become available under your profile directory. Then, tells firefox to load libraries from there and it will start.
guix package -i glibc glib gcc gtk+ libxcomposite dbus-glib libxt LD_LIBRARY_PATH=~/.guix-profile/lib/ ~/.guix-profile/lib/ld-linux-x86-64.so.2 ~/firefox_directory/firefox
Also, it seems that running icecat and firefox simultanously works, they store data in ~/.mozilla/icecat and ~/.mzoilla/firefox so they are separated.