Will Shrinking and lowering the high water mark cause issues in OLTP systems

newb here, We have an old Oracle 10g instance that they have to keep alive until it is replaced. The nightly jobs have been very slow causing some issues. Every other Week there is a large process that does large amounts of DML (deletes, inserts, updates). Some of these tables have 2+ million rows. I noticed that some of the tables the HWM is higher than expected and in Toad I ran a database advisor check that recommended shrinking some tables, but I am concerned that the tables may need the space for DML operations or will shrinking them make the process faster or slower?

We cannot add cpu due to licensing costs

If you are accessing the tables with full scans and have a lot of empty space below the HWM, then yes, definitely reorg those (alter table move). There is no downside, only benefit. But if your slow jobs are using indexes, then the benefit will be minimal.

Don't assume that your slow jobs are due to space fragmentation. Use ASH (v$active_session_history) and SQL monitor (v$sql_plan_monitor) data or a graphical tool that utilizes this data to explore exactly what your queries are doing. Understand how to read execution plans and determine whether the correct plan is being used for your data. Tuning is unfortunately not a simple thing that can be addressed with a question on this forum.

In general, shrinking tables or rebuilding indexes should speed up reads of the table, or anything that does full table scans. It should not affect other DML operations.

When selecting or searching data, all of the empty blocks in the table and any indexes used by the query must still be read, so rebuilding them to reduce empty space and lower the high water mark will generally improve performance. This is especially true in indexes, where space lost to deleted rows is not recovered for reuse.