How to nice/renice all the future processes of a user

One of my server's users needs to launch CPU and I/O intensive jobs that are also quickly done (usually less than 30 seconds). They can be started by a user’s cron, a user’s shell or even Apache, PHP or MySQL processes forked/launched from root’s processes (MySQL and Apache run as root then forked processes set themselves as the corresponding user). Is there a way to nice an entire user and all its future processes owned by this user? **EDIT** Added more precision about the kind of processes targeted