Or would you also expect individual letters to fade in when you enter some text? That’s just going a step too far, imo, and conveys absolutely no new information to the user about where the letters came from. I don’t perceive any ambiguity that the animation helps to remove, nor does the transition feel startling to me. The fact that a check mark appears without any delay after you've checked the box seems like an advantage to me. Makes you feel like either the computer or the display can’t keep up with what you’re doing – or, worse, thinks you could use some distraction. To provide another example, Windows used to (possibly still does?) make the background color for highlighted menu items slowly fade in and out as you move the cursor between items. Technically nothing that would necessarily slow you down, but it can get on your nerves eventually and leave you with a feeling of slightly less control. Like using an old TFT display with a low pixel response time that would result in ghosting artifacts.

I do think that they make the UI feel a bit laggy though.

You are right in that they are far from the worst (although the search fields with a centered 'Search' label that would move to the left after being selected were pretty awful).

Snagit 2018 issues with mojave 10.14.2