Will I notice performance increases if I switch to Lubuntu? In what areas will I see the most significant changes? And in what areas am I unlikely to notice any?
Would switching to Lubuntu be indicated for an older computer with <= 1gb ram?
Lubuntu is radically faster than most other desktops and requires very little RAM. It's the same operating system, though. It's just a different desktop. You can install the LXDE desktop in Ubuntu by following this link: https://apps.ubuntu.com/cat/applications/natty/lxde/
Note that this will not be the same as Lubuntu. Lubuntu contains Chromium instead of Firefox, etc. Installing LXDE will not install those extra packages, only the desktop. You can then choose to login with LXDE by pressing the button next to the password field when you log into Ubuntu. Note that Lubuntu also uses some nice themes and things that aren't part of the LXDE package.
To install the entire Lubuntu package, use this link: https://apps.ubuntu.com/cat/applications/natty/lubuntu-desktop/
A short answer is YES: LXDE is a lighter-weight environment, also Lubuntu uses lighter equivalents of some programs (file manager etc.), absence of 3D-effects means there's no need for Compiz or other compositing manager - this all results in lower memory usage.
Lower memory usage means there's more space for your programs to run, meaning they'll load faster and in many cases will run faster.
Also, more free memory means more space for disk cache, so some file operations may improve too.
Areas where you will unlikely notice any improvements:
I would definitely recommend Lubuntu or some other lightweight distribution on a computer with <=1gb RAM (actually, <=2 Gb).
Lubuntu is designed to be a Ubuntu-based system that works well on hardware that might be too outdated or slow to run a full fledged Ubuntu/gnome desktop - however, I know many people who use it as their main desktop (including myself).
To install Lubuntu, use the Ubuntu Software Center..
Now for the difference between
Lubuntu uses the LXDE desktop, which runs lighter on the graphics than GNOME (especially GNOME 3), KDE, and even XFCE. On older systems with low RAM and lower-end graphics, it will dramatically improve performance. The Lubuntu distro is also more lightweight than Ubuntu in terms of install space and resource-hungry applications. It was designed for low-end and outdated hardware, so the included apps are also resource light, like Chromium instead of Firefox, and AbiWord and Gnumeric instead of the full LibreOffice suite. For "normal" users, those who browse the web, write documents, etc. Lubuntu has everything they typically need. If you're intending to do video editing or more intense processing, then you're probably using hardware that can handle Ubuntu without problems.
According to the Lubuntu website, the base system requirements:
If you're using top-of-the-line or even currently run-of-the-mill hardware, there aren't likely any advantages to Lubuntu over the other flavors of Ubuntu; but if you're using older or lower end hardware, even netbooks, you'll probably appreciate the lighter interface, freeing up resources for applications. A computer with less than a gig of RAM would probably appreciate the extra free resources in Lubuntu, as my old laptop with 1.25GB runs much more smoothly and swiftly on Lubuntu than it did on Ubuntu or Xubuntu.
I would consider lubuntu much faster as compared to ubuntu. I did live testing by spinning up two Virtual machines on my desktop both with identical configuration with 512 MB RAM and 1 CPU.
Booting and installation time was almost same, but when it comes to opening multiple applications such as opening multiple tabs on browser Lubuntu really surpases Ubuntu in speed due to its light weight desktop environment. Also opening terminal was much quicker in Lubuntu as compared to Ubuntu.
So my vote goes to Lubuntu if you have older processors and less RAM, most likely your old PC then go for Lubuntu, however, if you have newer PC with good hardware then go for Ubuntu to have its rich GUI features. Hope this helps.