Any 5Mbps will mostly only be enough to stream 1 x 720p video without buffering [x.264 - high profile, high bitrate mp4]
or, 3 x 480p without buffering - baseline/constrained baseline profile, low bitrate mp4
For 1080p you will need 10Mbps.
5Mbps will buffer on 1080p, unless they cap u higher at 6 - 8Mbps.
2 x 720p sometimes can, sometimes cant, depends on how the uploader encoded their file and how youtube post-process it.
Some 720p files are large, some can be really small. For standardized VEVO music video @ 720p, nope, most of the time 2 computers will buffer @5Mbps anywhere in the world. The problem is not with unifi, it's just the way the video is encoded.
If you want more accurate theoretical calculation. Take Gangnam Style 720p for example. The file is 88MB(MegaByte)
CODE
5Mbps is equiv to 0.625MBps
88MB / 0.625MBps = 141 seconds (2 min 21sec)
The video is however 4min 13 sec long. So with 1 person loading, it's fine. The whole vid will load in 2min21s.
But if 2 person is loading, assuming the available 5Mbps is equally divided into 2. Hence we will multiply the loading time by 2.
(2 min 21sec) x 2 = 4min 42sec ...with 2 person loading
Since it will take you 4min 42sec to load a 4min 13sec video. U will get buffering, by at least:
4.42 - 4.13 = 0.29 = 29 sec of buffer/wait time.
Of course the loading time will be longer than what i stated due to IP transit overheads 10%+/-
Check from : Copy+Paste Official vid link
http://www.youtube.com/watch?v=9bZkp7q19f0&feature=g-music to
http://keepvid.com/ Allow the javascript to run, and it will present u the list of video, 240 - 1080p with their respective size.

1080p if u pause it for 20sec then start play, can play smoothly (but also note that sometimes youtube feel very fast.. other times slow.. actually other times even 480p takes time to load.. maybe tm international link very full used that time..)
so maybe not very accurate to measure bcoz its not contant.. tm always variable performance not the same ...