Oracle Database Discussions

Hi
In our office on 10-12 PC's we are using tool/software made from Oracle Forms & Reports 6i .The main task of this software/tool is to insert & update records into DB which maintained on UNIX server separately. There are total 25 users on Oracle Database .
Some of the user have large no of records to insert into DB & some have less. We are inserting records into DB on monthly basis. During each month end task of insert & update is at peek and what is happening is that some users which have large records to insert & update are taking enormous time than users which have less records & these large record users are almost hanging there for hours affecting whole DB's performance & slowing other office tasks.
Separate tablespace are allocated for each user and the size of tablespaces for users with large records is almost double of those with less record .Apart from month end during normal days the speed and performance of large record user is also less than the less record user but it is not affecting other office tasks. Now my question is how to increase the speed and performance of user with large records and also how to avoid effect of this on other office work do I need to change my DB's parameter for this?
Please guide!!