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• Storing sessions in memory will always be faster than storing sessions in Redis, as long as there is sufficient available memory to store the sessions. The difference in response time depends on the application (profile size being the largest factor), but also on a number of settings (see Tuning Advice below). In general, the difference between storing sessions in memory versus storing sessions in Redis should be on the order of a few tens of milliseconds, thus imperceptible to the user.
• When  storing sessions in memory, the amount of available memory has a much larger impact when compared to storing sessions in Redis. When storing sessions in Redis the combination of available memory and load are the deciding factor, since Redis has to keep a session in memory only for the duration of one request. 
• Performance degrades much more quickly when storing sessions in memory. When almost all available memory is used up, the response times will increase dramatically as the garbage collector uses lots of CPU cycles trying and failing to free up memory. In contrast, storing sessions in Redis yields a smaller performance degradation rate. 

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