After upgrading from 1.4.2 to 1.5.0 Blue Ocean UI gets stuck while running a big declarative pipeline job.
The pipeline job has 4 stages running in parallel. Each stage contains 28 steps. 8 steps (out of 28) in each stage launch other pipeline job.
When execution reaches somewhere about step 15-20, UI navigation between parallel stages becomes slower and slower, and at some point of time UI gets stuck for tens seconds and even minutes, so it is impossible to use it at all.
I suppose that the issue can be related to the new feature UI for downstream jobs launched with 'build' step
Downgrading to Blue Ocean 1.4.2 resolved the issue