I found a definition of processing speed (it has a lot of references, too):

Originally Posted by Johns Hopkins webpage
...processing speed [is] the ability to rapidly and efficiently respond to basic stimuli. Processing speed is typically defined as speed of completion of a task with reasonable accuracy. Processing speed tasks include tasks such as quickly associating numbers with symbols (including components of IQ tests), searching for and responding to specific targets, as well as rapid naming of visual stimuli. Thus, processing speed may be a more fundamental cognitive process that underlies the efficiency with which one can read and write.