Continuous Integration is often misunderstood as being a build server, but it's actually the practice. The main tenet is that:
Everyone commits to the mainline every day
As Branches are cheap in Git, spinning up a branch for short-lived activity is an easy practice to do. The mainstream Git-based tooling utilises this to create the Feature Branch/Pull Request model.
However, Pull Requests can encourage developers to hold the work back until they've done everything. This essentially ends up creating long-lived branches, which is contrary to lean understanding:
Furthermore, consider that Pull Requests can be psychologically unsafe in Command and Control working environments
Models which hinder Continuous Integration, such as the Pull Request model, will slow down teams (Pull Requests can hinder Continuous Integration).
Therefore, Pull Requests can hinder Continuous Integration by destabilising the overall team flow.
This is why Pull Requests can hinder Continuous Integration
As Small increments act as save points - this development practice avoids huge merge conflicts incurred from the pull request model - Pull Requests encourage rework and therefore Pull Requests can hinder Continuous Integration
Pull Requests can hinder Continuous Integration but they also slow down the overall project through how rework is handled.