Jobs which were initiated during other login sessions and which are still running cannot be brought to the foreground. A background job or a stopped job may be brought to the foreground with the command fg. There are two ways to refer to a particular job. This can be used to terminate stopped and background jobs from the current session. The effect of background jobs on system performance depends upon the priority at which they run and how much RAM they require to run.
Large jobs can consume immense amounts of RAM which can severely limit system performance.
If the program requires too much RAM, the CPU will be wasting a lot of time swapping memory to and from disk rather than executing the jobs it needs to. The other variable which affects system performance is job priority. If your jobs run at low priority, they will be done at the system's leisure and system performance will not be affected. If they are run at normal priority, they will increase the competition for resources and performance may be degraded.