Are you caching a keyed HMAC context? Given the data sizes, it ought to be at least one call of the SHA transform function to set the key, and one to produce th

Are you caching a keyed HMAC context? Given the data sizes, it ought to be at least one call of the SHA transform function to set the key, and one to produce the hash. They keying operation isn't dependant on the data and its result can therefore be reused.