Performance notes

Here are some general pieces of advice on performance:

• Use one graph.addNodes() instead of multiple graph.addNode() (same for edges), as well as graph.removeNodes() instead of multiple graph.removeNode().
• Text is really expensive in term of graphics memory. For reference, one character uses as much space as a node shape. Consider disabling texts if you run out of graphics memory.
• Big elements on screen takes an order of magnitude longer to be displayed: adjustments such as decreasing the size of nodes, or zooming-out can make the interactions much smoother.