Many web/network applications performance is dominated by IO concerns rather than raw number crunching. There are some unique cases where you can benefit from such optimisation, however they are usually quite rare and quite small in comparison to focusing on other areas like DB query optimisation, caching etc. Dropping down to assembly is a lot of pain and very expensive in terms of development/ maintenance for what is likely to be a tiny percentage of the possible benefits. At the very least it is worth testing and benchmarking to find out what is actually slow in your application design before working on optimising.
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