Your tool is faster compared to LC's, which I think is due to the static chart image.
However, I would still like to see the chart image to reflect the latest data from LC. Therefore, perhaps generating the chart image once daily would be enough. Or weekly? Not sure how often LC updates the JSON data.
Glad you noticed the speed. We cache the entire JSON file for 24 hours to speed up processing so we don't even need to wait for LC to give us the data.
If we do generate the chart dynamically we would want to keep the speed so we would probably do it daily too. Having 24,000 points in html/css might slow things down even if the JSON doesn't need to be processed. I will have to play with it to find the best combination of speed and accuracy.
Feedback is good so far so it is probably worth the time to improve it.