OS/390 Batch FTP causing performance problem

When we run a batch FTP job using very large files, it affects the rest of our system. I.E. Other batch JOBS, TSO, and worst impact is to CICS response. I read somewhere that to help smooth performance you need to reduce the size of the packets sent/recieved to FTP but, I don’t know where to set packet size or buffersize. Also is there performance group settings that will help?