  The pull-based development is widely adopted in modern open-source software
(OSS) projects, where developers propose changes to the codebase by submitting
a pull request (PR). However, due to many reasons, PRs in OSS projects
frequently experience delays across their lifespan, including prolonged waiting
times for the first response. Such delays may significantly impact the
efficiency and productivity of the development process, as well as the
retention of new contributors as long-term contributors.
  In this paper, we conduct an exploratory study on the time-to-first-response
for PRs by analyzing 111,094 closed PRs from ten popular OSS projects on
GitHub. We find that bots frequently generate the first response in a PR, and
significant differences exist in the timing of bot-generated versus
human-generated first responses. We then perform an empirical study to examine
the characteristics of bot- and human-generated first responses, including
their relationship with the PR's lifetime. Our results suggest that the
presence of bots is an important factor contributing to the
time-to-first-response in the pull-based development paradigm, and hence should
be separately analyzed from human responses. We also report the characteristics
of PRs that are more likely to experience long waiting for the first
human-generated response. Our findings have practical implications for
newcomers to understand the factors contributing to delays in their PRs.
