The Window Manager - or for most modern Linuxes the Desktop Environment - makes a huge difference. The more bells and whistles (especially 3D ones) your desktop uses, the less resources are left to actually run programs.
Use a lightweight Desktop Environment like Xfce or LXDE instead of GNOME KDE or Unity (the standard for Ubuntu 14.04) - ie. install Xubuntu (which uses Xfce) or Lubuntu rather than Ubuntu (Unity) or Kubuntu (KDE)... or if you have it installed, install lxde
or xfce
and set it as default Desktop Enviroment.
Even more lightweight would be a pure Window Manager like icewm, blackbox, windowmaker or mwm (Motif Window Manager). A "Desktop Environment" is basically an advanced Window Manager (advanced meaning that it has desktop-switching, background-pictures, task-bar and so on), but also with a set of application with a similar and consistent look and feel (eg. how KDE comes with a whole set of KDE-applications for things like editing files, surfing the net, instant messaging etc - all with the KDE look).
It's no problem starting an application "belonging to" a Desktop Enviroment (like kopete
, the KDE instant-messaging client) even if you're not running KDE but runs another Window Manager instead. Note though that some parts of the Desktop Environment it belongs to may have to be started, so some of your gain by using a lightweight alternative may be lost.