Why is this job slow?
Before asking why the GPU is slow, ask which part of the job became slower. A timeline gives you somewhere specific to investigate.
Find the added time
Prepare 20 · calculate 40 · communicate 10 · write 5
Prepare 20 · calculate 41 · communicate 30 · write 5
Make the comparison fair
Keep input, model, precision, batch size, concurrency, and versions comparable. Separate first-run loading from steady operation.
In the fictional diagram, total time rises from 75 to 96 seconds. Communication accounts for 20 of the 21 added seconds.
That is the first investigation target, not proof of a bad network. A participant arriving late can also make others wait.
Test one explanation
Compare placement, link health, software changes, and phase timings. Write down what evidence would disprove your leading hypothesis.
Preserve logs before intervention. Resets and disruptive diagnostics require approval; they can interrupt work and erase evidence.
Try it yourself
Would a faster GPU explain or repair the communication increase?
Check your answer
No. It might shorten another phase while leaving the regression untouched. After a fix, repeat the original workload and check correctness as well as duration.