Does uttering the word of sincerity (Kalimat al-Ikhlas) require instruction from a sheikh like "Muhammad Salih al-Munajjid"? And would his instruction have a stronger effect on the soul? And is it permissible to speed up the dhikr after starting it slowly, or should the dhikr always be at a constant, slow pace?

The intent of reciting "Lā ilāha illā Allāh" (There is no god but Allah) was misunderstood. Recitation is typically for the dying, whereas a living person pronounces it themselves to embrace Islam or to attain its virtues. There is no specific manner for pronouncing it, whether quickly or slowly. What is prescribed is contemplation so that the heart and tongue are in agreement. While the reward is more complete with contemplation, this does not negate the establishment of reward for one who recites it without contemplation. One must be wary of obsessive doubts (waswasa) and focus on the meaning of the word of monotheism and its conditions, which include knowledge, certainty, truthfulness, sincerity, acceptance, submission, and love.