Is there a lightweight way to snap windows in Lubuntu?

Super + arrow

Super + ← and Super + → split the screen horizontally.

Super + ↑ and Super + ↓ split it vertically.

Using a lightweight window manager

You could use the lightweight compositing manager used by XFCE called xfwm4

This will give you basic windows snap.

N.B. if you dont like the default "change workspace when dragging a window off the screen" capability then untick the very last option shown in the picture.

Getting Tiling to work

However - if you want the "tiling-snap" of compiz you will need to get your coding-hands dirty because this is work-in-progress.

According to this article - there is a potential patch available to enable windows tiling (aka windows snap) for the xfwm4 compositing manager.

Through the following Q&A I have packaged this tiling patch into my PPA - instructions for installation can be found in the linked answer.

There is a great you-tube video showing this in action.

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How to use xfwm4 in lubuntu/lxde

To use xfwm4 as your compositing manager instead of OpenBox.

In Desktop Settings - Advanced options change the window manager to xfwm4

Now in terminal:

cd ~/.config/lxsession
mkdir Lubuntu
cp LXDE/* Lubuntu/

Install the compositing manager:

sudo apt-get install xfwm4-themes

Logout and login for the xfwm4 compositing manager to take effect.

Change the style via xfwm4-settings

I personally use quicktile.

It allows you to snap windows in different regions of your screen based on keyboard inputs (e.g. Ctrl+Alt+1 to get the window to the top-left corner). It can be run as daemon on any X11-based desktop and only requires python as a dependency.

As far as I can tel, it does not have a drag-and-snap function, but your question did not seem restricted to mouse-only behaviour.