There are many details in realizing replicas, when considering efficiency. Consistency is maintained in the whole system to ensure the same eventual result at least. Different levels of consistency should be compared carefully. Fault tolerance is another important point. Either the fault of master database or that of slave one should be recovered. High availability means that using the same hardware, it can provide as much service as possible.
When the idea of parallelism combines with network implementation, many new problems come out.
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Another name for "scale up" and "scale out" is vertical scaling and horizontal scaling. A detailed explanation is in wiki.
Is consistency between the master and slave databases a bottleneck of any sort? I assume that since there is mostly static content on a webpage, then keeping databases consistent is not difficult and can be done without much interruption to site availability.