Comment 3 for bug 2015979

I don't think this is so much of a bug in Lubuntu as it is a limitation of most VM software in general.

VM software is generally very good at virtualizing a computer's CPU and most other "normal" components (sound, motherboard, etc.). However, graphics hardware is much more difficult to virtualize most of the time due to limitations of the GPU hardware (only some GPUs allow you to "split" one physical GPU into multiple virtual GPUs), and so virtualization software will usually only provide relatively basic graphics capabilities to the VM by default.

XScreenSaver relies on having graphics hardware to display screensavers. When that hardware isn't available, it falls back to using the CPU to render the screensavers. And since the VM software doesn't provide good graphics acceleration to the VM, XScreenSaver puts a large load on the CPU, causing the resource usage spike you're seeing.

The best way to get around this is probably to just disable the screensaver - you should be able to do this by opening the XScreenSaver settings (by going to the Application Menu, searching for "Screensaver", and then clicking "Screensaver). There's a menu on the left-hand side near the top where you can select "Disable Screen Saver", if I remember correctly.

The screensaver is (at least in all existing versions of Lubuntu) an important part of the Lubuntu experience, and disabling it by default in existing releases of Lubuntu would be a feature-breaking change that we would probably not be allowed to make (as Lubuntu is an official Ubuntu flavor and thus has to abide by Ubuntu's rules when it comes to how we do things). And the soon-to-be-released Lubuntu Lunar release has already passed Feature Freeze, so we're probably already stuck with the screensaver enabled by default there. However, we may be able to consider disabling the screensaver by default in a future release of Lubuntu, if we determine that the costs of the screensaver outweigh the benefits of having it enabled by default. For now, disabling it manually is probably the best solution.