Mathjax often renders very slowly for me in the preview when I type up a question. I always have to stop typing and wait for 5 seconds. A similar complaint can be found here. The answer there says that the issue is fixed, but it isn't for me. I have made a screen recording of the issue. I tried using a different browser, but that did not help. It doesn't happen every time.
Edit. I should mention that I only have the problem when I write my own question. When I answer the question of another person then the formulas render immediatelly in the preview, so I do not think that it is a problem with my computer.