https://github.com/steemit/steem
The current minimum time interval allowed between comments is 20 seconds. This was firstly designed in order to make it harder for people to spam the Steem blockchain. Right now, because a malicious user could make multiple accounts, it can spam as many comments as he wants, being limited only by the bandwidth that he has, which is determined by the amount of Steem power that the corresponding accounts posses at the moment of the spam. In this case, having the comment limit interval of 20 seconds became irrelevant, as it has only the effect of making it harder for people to communicate.
Reducing the minimum comment interval from 20 seconds to 3 seconds was requested in this issue and it was included in the development section for the Hard Fork 20.
The number of 3 seconds was chosen as a lower limit of seconds between comments, because it is the time at which blocks are produced on the Steem blockchain, and it was not desired for one author to have two or more comments included in the same block.
The development of this feature was made on the branch with the name 2019-removal-of-20s-comment-limit which was successfully merged on the pull request with the number 2439 and it will be live with the Hard Fork 20.