[Google Chromecast] bhiga:
Yes, speed4cast measures the speed between Chromecast and the Internet, but you cannot measure the Internet connection speed independently from the wireless connection on Chromecast, because the Internet traffic is flowing through the wireless connection.

So, if the reported upload/download speed is anything LESS than your Internet connection speed, then the Wireless connection* is slowing things down and the reported Internet connection speed is really your wireless connection speed.

For example, I have 50 Mbps down, 20 Mbps up. If I do an Internet speed test on a computer, I see 40+ Mbps down speed.

Yet, my Chromecast is only 8 Mbps down?

If I do the same test on my phone connected at 2.4 GHz I get much better results.

So, the wireless connection between the router and my Chromecast is limiting its transfer speed.

* if your wireless AP is not your router or there are other network segments between the Internet connection and the AP, they may be causing the slowdown instead of the WiFi connection. Also, if your router/AP is slow, the wireless devices may be connected at a high rate, but transfer to/from will never reach the maximum because the router/AP is slowing things down.

Checking the wireless speeds on your other devices doesn't mean anything because those other devices are not where your Chromecast. The closest you can get is maybe to put your phone/tablet as where Chromecast is, which is usually right up against the back of the TV. You're almost guaranteed to notice the speed will drop significantly.

As I mentioned earlier, tests with dedicated streaming video services like YouTube, Hulu, etc will not give you an idea of your wireless transfer speed unless they provide visual feedback for the connection quality.

They are designed to transparently handle a wide range of connection speeds, unlike screen casting which essentially says "I need this much bandwidth, or it's not going to work correctly."

If you have Netflix, you can try their speed test video.

This article talks about it. Note that on my Chromecasts with optimal wireless, I get up to 5300 kbps which appears to be the max.

On suboptimal wireless I get far less.

http://www.wired.com/2014/06/netflix-streaming-test/

Given your other devices have good WiFi performance, at least we can rule out your router slowing things down.

Your phone may be able to send data to the AP very quickly, but if Chromecast cannot receive that data fast enough because it has a poor wireless connection, then that doesn't help.

The data flow for screen casting is this:

Phone/Tablet <--A--> AP/router <--B--> Chromecast

Segment A is great, but segment B is what is suspect. Only hard numbers will tell.

speed4cast measures

Internet <--C--> AP/router <--B--> Chromecast

And you've already said your Internet speed is more than adequate (it should be unless you're on public/hotel connection, dialup, ISDN or repeating another WiFi connection).

B is the common part, and eliminating C, speed4cast will give us a measurement of B.

So run speed4cast on your Chromecast and tell us what it says on the screen.

If it confirms that your Chromecast has a good bandwidth connection, then it's something else in play. But in most cases interference from the TV makes Chromecast have less than optimal wireless speeds, and that's why you see some people complaining about streaming quality, because the streaming server is downgrading the bitrate (and hence quality) to compensate.

Screen casting doesn't do that, so if your Chromecast's wireless connection can't sustain the required speed you'll get blockiness, low framerate, jumps, or disconnection.