For a project we needed to import a large amount of data into a RAC database.
A certain partitioned table with some 4 million rows took ages.
Grid Control showed a large value for the log buffer waits.
So I increased the FAST_START_MTTR_TARGET to reduce the number of checkpoints which worked almost immediately.
Another possibility could have been to increase the log_buffer size (14 M to 32 M) as the ADDM suggested. Maybe I do this as a structural change. Right now changing the parameter on the instance where the import happened solved the problem.