Descobri que a marca de velocidade garantida são unidades que afirmam "readyboost enabled". O recurso readyboost do Vista requer alta velocidade, portanto, se a unidade tiver isso, então, se deve ser rápido.
As especificações mínimas para uma unidade habilitada para o readyboost são:
(De acordo com a página da Wikipeda no readyboost )
For a device to be compatible and useful it must conform to these requirements:
The removable media's capacity must be at least 256 MB—250 MB after formatting. Windows Vista x86 is limited to using 3.5GB (Vista x64 can support up to 16GB); this restriction has been removed in Windows 7.
The device must have an access time of 1ms or less.
The device must be capable of 2.5 MB/s read speeds for 4KB random reads spread uniformly across the entire device, and 1.75MB/s write speeds for 512 KB random writes spread uniformly across the device.