Forgot to mention it but it has to do with the lack of optimization of the Retina resolution. It's not that the hardware can't handle it (which is why it lags with smallest and largest resolution options)
I disagree. This has clearly to do with image data. When you disable the website images, the lag stops. That website has only a few dozens images, drawing them is very cheap. Its something with website setup which messes up the performance. I remember there was a bug in Chrome where the images would not fit into the cache and the browser had to decompress/resize the data for each displayed frames - performed on the CPU, this is a quite expensive operation. The current lag might be a similar problem. For some reason the images do not get uploaded to the GPU as fast as it could process it.