A Class rating of a card is the minimum write-speed afforded by the card. For the purpose of performance assurance, it is safer to use the manufacturer class rating rather than write-speeds obtained on specific writing devices. I have seen SD Card write speeds vary depending on which Card Read/Writer interface was used.
While Class ratings are in MBps (Megabytes Per Sec), video recoding rates are Mbps (Megabits Per Sec). The difference is a factor of 8.
Typical DVD quality video (480p) is at 11 Mbps (Raw stream) with up-to 6Mbps dedicated to Video stream (rest split among audio, subtitle, sync etc.). This equates to approx. 1.375 MBps write rate (a Class 2 card should do it).
Typical BlueRay quality video (1080p) uses a 48Mbps (max. data rate is 54 Mbps) of which up-to 40Mbps is dedicated to video stream. This equates to approx. 5 MBps write rate (a Class 6 card should do it).
A Class 10 card provides sufficient write speeds for Full-HD videos to be recorded to the card in real-time without any buffering and frame-drops. The ample speed also ensures that parallel File Allocation Table operations can occur without interrupting the video stream long enough to drop frames.
SanDisk extreme class 10 speed is up to 32MBPS...even SanDisk ultra class 4 have speed up to 15MBPS
Soo I think 10Mbps is slow...even my mobile records 720p on 14000KBps or around 14MBPS