1) Make redologs bigger so that average time gap between redo log switch is 20-30mns.
2) Make sure redo logs are not created on disks of large stripe size or on RAID5 disks.
3) Try to create raw redo log files
4) Be sure the prcoess paramter is not much higher than required.
5) Be sure redo copy latches are not much higher than required.
6) At OS level try to find if LGWR process has a I/O wait or bottleneck.