Ballpark figure for a HDMI to VGA decoder is around 200 mA at 5 volts. That's a watt. For such a small device, you're going to expect that to make it fairly warm.

Adapters are available to connect VGA monitors to laptops that have HDMI or DisplayPort. NPD In-Stat is forecasting shipments of devices with DVI, HDMI and DisplayPort to pass 2 billion by 2015.

An Apple spokesperson told The Verge that Lightning to HDMI and Lightning to VGA cables “will be available in the coming months.” by Sean Hollister Sep 13, 2012, 11:07 PM UTC ...

Thunderbolt Gen 1 and 2 don’t support USB 3.1, Type C or even 3.0 for that matter. An adapter wouldn’t have fixed this problem. However, Apple is releasing Thunderbolt 3 later in 2015.