What are you running? Hardware? Distro?
I've recently been using eOS, and have also recently used Ubuntu; both of which are running quite quickly on 6-7 year old hardware.
If you want something stable that doesn't crash easily I recommend Debian.
If you want it to run fast don't let your desktop environment use up all your CPU power. Don't use GNOME 3 (or if you do run it in "Classic mode"). Don't install unnecessary special effects like Compiz.
I recommend you install it on a reliable and fast storage device, such as an internal hard drive. If you install it on a USB stick or an external hard drive make sure that it is fast enough and that you get full speed (I know there has been bugs that made USB 2 and 3 run at the same speed as USB 1).
If you mean that Firefox is being slow you could try if clearing the history helps. I have noticed that Firefox can get slower and slower if you don't clear the history once in a while. Ctrl+Shift+Delete will bring up a dialog window. Change the time range to "Everything" and check most of the boxes (I often leave the cookies to avoid being logged out from everywhere) and then press "Clear now".
Well, If anyone is looking for fast and stable linux distro, then one should try Debian[https://www.debian.org/[/url]]. I am using Debian since long and it has proved to be good and stable and it never crashed. The CPU usage is not high when you use XFCE OR MATE and any other light desktop environment.
I use Debian Wheezy with mate desktop environment and RAM usage on boot is just 100 to 125 MB.
So, I would say that go for Debian stable branch if you need a fast and stable linux.