The "checkpoint not complete" messages are generated because the logs are switching so fast that the checkpoint associated with the log switch isn’t complete.
Oracle suggests that you might consider setting archive_lag_target to zero to reduce "checkpoint not complete" messages:
alter system set archive_lag_target=0 scope=both;
Overall Oracle performance can be dramatically improved by increasing the log sizes so that logs switch at the recommended interval of 15 to 30 minutes.
Identify the current size of the redo log members from v$log, record the number of log switches per hour and increase the size of the log to allow Oracle to switch at the recommended rate of one switch per 15 to 30 minutes.
For example, if the database redo log size is 1 megabyte and you are switching logs every 1 minute, you will need to increase the log size to 30 megabytes in size to allow it to switch every 30 minutes.
You need to ensure that the on-line redo logs don’t switch too often during periods of high activity and switch often enough during times of low processing workloads.
This should reduce the delays from the checkpoint not complete errors.