What exactly do you mean? Because your question could be asking - 1) does using JS slow average page loading times? or 2) is JS actively slowing THE INTERNET down?

If 1 - it could be; it is hard to pinpoint the cause exactly. The average page loading time is increasing (http://marketingland.com/retail-website-load-times-continue-...).

If 2 - I don't think so; Bandwidth is increasing, and average internet speed is also increasing (http://www.akamai.com/dl/akamai/q2_2013_soti_infographic.pdf)