Respondents who need special consideration (such as a dyslexic candidate) have to be accounted for during the assessment process. Dyslexia is a developmental disability whereby an individual has difficulty with written language involved with spelling and reading. It is therefore quite likely that a dyslexic respondent will display performance deficits when completing aptitude tests, especially verbal reasoning tests. It is best practice to send out an invitation letter to all respondents and ask them to advise you in advance if they have any special needs. At that time you may consider how to approach this situation. Most test publishers will NOT allow extra time for completion. This is because the test then becomes unstandardised and no longer measures what it was designed and tested to measure. A number of publishers recommend either using tests specifically designed for those with dyslexia, or to ask them to undergo the same process as all respondents and interpret their results in light of the dyslexia. This may mean shifting their sten score by 1-2 points.
This issue is less material for personality assessment wherein the test is untimed. It would be appropriate to simply give as much time as is necessary (within reason) for the candidate to complete the assessment.