If I enable the change tracking metadata for a table, will that have an additional cost?
ALTER TABLE … CHANGE_TRACKING = TRUE;
And with this enablement DML operations will slow down?
Extra storage costs for using this feature will be very low/negligible in most cases:
Either option (CHANGE_TRACKING or STREAMS) adds a pair of hidden columns to the table and begins storing change tracking metadata. The values in these hidden CDC data columns provide the input for the stream metadata columns. The columns consume a small amount of storage.
The impact on DMLs is also minimal. It's like having two additional columns in the table (one varchar and one number).
There won't be any other costs as it does not need any extra maintenance.