Вот захожу на официальную страницу http://lubuntu.net/ и читаю как английским по белому написано System Requirements
Lubuntu is a good operating system for many old computers, but not for all of them. Some computers have too little horsepower or memory. A rule of thumb is that the computer should not pre-date 2000. The desktop images for lubuntu 16.04 requires a DVD / USB device to write it to, the alternate images are for those with have non-standard setups or Low RAM.
For advanced internet services like Google+, YouTube, Google Docs and Facebook, your computer needs about 1 GB of RAM.
For local programs like LibreOffice and simple browsing habits, your computer needs about 512 MB of RAM. See minimal for systems below that RAM.
The minimum specification for CPU is Pentium 4 or Pentium M or AMD K8.
Older processors are too slow and AMD K7 has problems with flash video.
Graphics chip / card
Nvidia, AMD/ATI/Radeon and Intel work out of the box, or the system can be tweaked to work fairly easily. You can get help at the Ubuntu Forums. With such graphics, or if you don't know, try the current Lubuntu version.