If you have a Core 2 Duo 2.8 Ghz, or better or any AMD equivalent you will be able to compile Gentoo software and continue to work with your PC almost normally, but if you are working on a Celeron, or any other old PC, you will find your computer really slow, while installing new software on Gentoo.
You can use nice to reduce the priority of emerge/portage and let some available CPU cycles for your normal work while compiling. To do this, we will edit the /etc/make.conf file with your favorite text editor mine is vim
sudo vim /etc/make.conf
Then, add this line to it
Here I am using the lowest priority, you may use some higher like 13 or 17.
Once emerge is running you can confirm the niceness it has, with htop</p>.