The very act of a pull request creates a bottleneck. It is a synchronous handoff to another member(s) of the team where approval to progress is mandated.
Pull Requests can hinder Continuous Integration but they also slow down the overall project through how rework is handled.
Rework is requested before the overall change is approved, which compounds the waiting time for the feature to be merged into the mainline
This acts as a disincentive to review the code, which then tends towards the usage of long-lived feature branches
The team's inventory increases as there is more work in progress
Quality is not built in, it's added after
The longer-lived the branch, the more merge conflicts arise as the branch is continually kept up to date with mainline