Here is an introduction. They are not SVG specific but you can build SVG elements in a document fragment, which is a lot faster than appending to the DOM, then append the fragment to the DOM all in one go.

See http://ejohn.org/blog/dom-documentfragments/ or google for an introduction.

Building strings and using innerHTML is a very ugly way to get performance, you should definitely try to using the document fragment approach as it has similar performance characteristics.