One thing I've done a lot is compiled optimized builds of the software packages on my system, to improve their performance and memory usage. However, this doesn't make as much of a difference as it used to. I originally started using Linux on a dated 450mhz pentium3 system, and I'm now using a homemade system with a 2.6ghz AMD Athlon X2 5000+ Brisbane processor. That, and I'm using Arch Linux, which already has its packages built with some optimization.
So, the performance gains of an optimized build aren't as great as they used to be. But I still do it from time to time with frequently-used programs, as I am usually a heavy multitasker. KDEmod is a branch of KDE that I've been using since I started Arch Linux. It is more modular than Arch Linux's official packages, and is optimized for Arch Linux as well. It also has its own build system, separate from Arch Linux's build system.
However, I was only able to get it working with the 3.x branch of KDEmod. With the new 4.x branch, I was never able to. And today, I decided I wanted to compile an optimized build of Okular. So, I go over to the KDEmod wiki to check their build system's instructions. It has changed, a lot.
It requires you to install a chroot environment, which is essentially a miniature linux installation that runs within the main installation, but without any virtualization. For the few gigabytes of extra hard drive space that this takes up, KDEmod's build system is simply not worth it. Especially if you're limited on hard drive space like I am. I only have a 40gb PATA drive that is at least 90% full, so a chroot environment isn't an option for me.
And, at this point, I'm wondering if it's even worth it to use KDEmod anymore. I don't see any real advantages in it for me anymore. I don't even use KDE as a desktop environment: I use LXDE. I just have KDEmod installed for a few of the applications.