if i had to guess COUNT(*) may load everything per row datawise vs using a index. but I'd have to run tests to confirm =)
First result in Google confirms that at least for Postgres: https://www.citusdata.com/blog/2016/10/12/count-performance/
Another one from stackoverflow: https://stackoverflow.com/questions/2710621/count-vs-count1-... with a citation:
> Don't be fooled by those advising that when using * in COUNT, it fetches entire row from your table, saying that * is slow. The * on SELECT COUNT(*) and SELECT * has no bearing to each other, they are entirely different thing, they just share a common token, i.e. *.