Could a failure of condensed inner speech be linked to slowed cognition?
I did not find a study trying to test your idea. I suppose your hypothesis could be tested by comparing the results Varieties of Inner Speech Questionnaire (VISQ) on the expanded/condensed dimension of participants with induced slower condition to a control group. 

McCarthy-Jones &amp; Fernyhough (2011) studied the correlations between the dimensions and psycho-pathological variables. They found the expanded/condensed dimension was not predictive of psychopathology. 

McCarthy-Jones, S., &amp; Fernyhough, C. (2011). The varieties of inner speech: Links between quality of inner speech and psychopathological variables in a sample of young adults. Consciousness and Cognition, 20(4), 1586–1593. https://doi.org/10.1016/j.concog.2011.08.005