performance mechanism

 I know that oracle server to increase performance make the following,

Modified or new data is not written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once.

but in the other hand 

Oracle writes every transactions from memory to redo log files as soon as possible

which is contrary mechanism !!!!!!!!!!!!

my question : when Oracle writes every transactions from memory to redo log files  is it causes a lot of disk access and effect on the performance.