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.