Quickly passing and parsing variable length float arrays
You can get some improvements by using JSON and moving the length increment to the back-end like so:

Nengo.SpaSimilarity.prototype.on_message = function(event) {
    var data = JSON.parse(event.data);
    var size = data.shift();
    while (data.length &gt;= size) {
        this.data_store.push(data.slice(0, size));
        data = data.slice(size);
        size = data.shift();
    }
    this.schedule_update();
}