Is it time to add a server

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http://serverfault.com – This question is about figuring out is horizontal scaling by adding servers is the next step or not. Here is the context: About 1400 concurrent users at peak time (between 11:00am-4:00pm) in Google Analytics Real-time stats. And it also show spikes of up to 75 pages per second and average per minute of about 1000. New Relic logs show average 25ms for each request (11ms in DB for 6 requests per page, 14ms in PHP). To me, that means each page will take 25ms and so, the server can sustain 400 pages per second of sustained traffic. Practically all long queries that could be cached in memory ar (HowTos)