How likely is it that rate-of-test-taking impacts on WM tests? Rate-of-test-taking (also known as r9) is one of the main subsets of processing speed, so is tested via e.g. the symbol digit coding task. I believe it is affected by e.g. depression.

How likely is it that rate-of-test-taking impacts on WM tests, the digit span test etc,?

It was pointed out in this answer that tests of WM should create a "bottleneck" so that other cognitive processes are implicated less. What about r9?
According to "The neuroscience of working memory capacity and training" by Christos Constantinidis &amp; Torkel Klingberg asserts that training can increase WM capacity, however this does not translate into better reasoning power:


  Ongoing research  aims to elucidate the extent to which the
  improvements  also translate to various other abilities, such as
  academic  abilities, that statistically correlate with WM capacity. 
  Indeed, transfer to reasoning ability has been reported to  be small,
  with inconsistent findings.


However, transfers to attention seem more promising:


  More-encouraging  transfer results are seen for attention: several
  randomized,  controlled trials, some of them including children with 
  ADHD, show decreases in inattentive behaviour in every-day life after
  WM training


They cite to support this:


  Klingberg, T. et al.  Computerized training of working  memory in
  children with ADHD — a randomized,  controlled trial.  J. Am. Acad.
  Child Adolesc. Psychiatry  44 , 177–186 (2005)
  
  Brehmer, Y., Westerberg, H. &amp; Backman, L. Working- memory training in
  younger and older adults: training  gains, transfer, and maintenance.
  Front. Hum. Neurosci.  6 , 63 (2012).
  
  Green, C. T. et al.  Will working memory training  generalize to
  improve off-task behavior in children with 
  attention-deficit/hyperactivity disorder?  Neurotherapeutics 9 ,
  639–648 (2012).
  
  Bigorra, A., Garolera, M., Guijarro, S. &amp; Hervas, A.  Long-term
  far-transfer effects of working memory  training in children with
  ADHD: a randomized  controlled trial.  Eur. Child Adolesc. Psychiatry
  http:// dx.doi.org/
  10.1007/s00787-015-0804-3  (2015).
  
  Conklin, H. M. et al.  Computerized cognitive training  for
  amelioration of cognitive late effects among  childhood cancer
  survivors: a randomized controlled  trial.  J. Clin. Oncol.  33 ,
  3894–3902 (2015)