aria2c is a ultra fast, multi-thread download tool which supoorts multi-protocols.
Portage (emerge) use wget as defualt download tool. Wget is greate, but aria2c is faster in my experience.
We can set aria2c as download tool in portage by modifying FETCHCOMMAND and RESUMECOMMAND variables.
Here is some introduction about these two variables:
(Manual of make)This variable contains the command used for fetching package sources from the internet. It must contain the full path to the executable as well as the place-holders \${DISTDIR}, \${FILE} and \${URI}. The command should be written to place the fetched file at \${DISTDIR}/\${FILE}. Also see RESUMECOMMAND.
(gentoo handbook) Fetch Commands When Portage needs to fetch source code, it uses wget by default. You can change this through the FETCHCOMMAND variable. Portage is able to resume partially downloaded source code. It uses wget by default, but this can be altered through the RESUMECOMMAND variable. Make sure that your FETCHCOMMAND and RESUMECOMMAND stores the source code in the correct location. Inside the variables you should use \${URI} and \${DISTDIR} to point to the source code location and distfiles location respectively. You can also define protocol-specific handlers with FETCHCOMMAND_HTTP, FETCHCOMMAND_FTP, RESUMECOMMAND_HTTP, RESUMECOMMAND_FTP, and so on.
According to above references, we can set FETCHCOMMAND and RESUMECOMMAND as follows:
FETCHCOMMAND="/usr/bin/aria2c -j 10 -d \${DISTDIR} -o \${FILE} \${URI}"
RESUMECOMMAND="/usr/bin/aria2c -j 10 -d \${DISTDIR} -o \${FILE} \${URI}"