Nokia Lumia 820 - What do you mean with fast random write access
performance?  Many memory cards specify a Speed Class rating
(e.g. Class 10). This speed class indicates the minimum
performance when recording video, and both read and write speeds
must exceed this specified rating. If we take “Class 10” as an
example, this “10” refers to the performance requirement that
the card can sustain a sequential non-fragmented write speed of
10MB/s. Unfortunately this Speed Class rating does not quantify
the performance of the card during random access operations.
With ‘random access’ we mean that the files are not being
accessed in a sequential manner, and as such the overall data
throughput with the memory card is compromised. The advice is to
use a card from a well-known manufacturer that is known to have
fast sequential and random access performance characteristics.