I use matrix job to split a big test task (severals hours of execution) in several sub-jobs.
This is handful because configuration is shared between each sub-job.
And I can specify a "hardware configuration requirement" for my tests using labels.
And it'es easy for me to add another sub-job, or to split one job.

But there is at least one thing that would make this use easier:

-> Show the average/last duration of each sub-job on the parent job page.

For now, there is only a link to each sub-job
And on each sub-job I can open last build to find its last duration.

Further more, it would be really great, to have a display like a classic view for the Parent job.
With each sub-job as a line and some informations on it on several raws.
(Last build time, last build duration, last status, lastfailure, etc)

That would be easier to track wich sub-job is too loooooong and slow down the whole build.