Generally speaking, writing to files is an expensive operation as any IO operation. This leads to other questions such as, how much am I logging to cause system slowdown? What file system am I logging to, is it shared, etc?
Have you looked at the logs? Are there exceptions being raised? How big does your log grow? And at what rate?
First, I would look at what's being logged, and the rate at which this is happening. Second, if logging is normal but mem utilization is high, I would bring logging down to further investigate.
Fist. Only log anything above or equal to a warning
config.log_level = :warn
without setting autoflush_log.
When the system is high, I would look at what "top" tells me as well as /var/log/syslog
An alternative utility to "top" is "htop". It gives you more info I find (easier on the eyes) as well as being highly configurable. http://hisham.hm/htop/