Why is encoding so slow compared to decoding?
For most kinds of compression, encoding is inherently slower than decoding. In the case of Speex, encoding consists of finding, for each vector of 5 to 10 samples, the entry that matches the best within a codebook consisting of 16 to 256 entries. On the other hand, at decoding all that needs to be done is look up the right entry in the codebook using the encoded index. Since a lookup is much faster than a search, the decoder works much faster than the encoder.
