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It's very important to understand that the default protocol used for the ping command is ICMP. The significance of this is that many routers will deprioritise ICMP traffic and therefore this will show (in a traceroute for example) as an increase in latency - potentially even a timeout. There is also the issue of firewalls dropping such traffic completely...
So yes, ping is a useful diagnostic tool - but don't misunderstand the significance of it: generally it's useful for detecting a large discrepency and/or determining if a particular IP location can be reached. No/slow response doesn't necessarily tell you anything of interest...
Specifically in terms of your question: the network layer of the OS is responsible for handling ping responses, and therefore if the server/computer doesn't have spare CPU cycles to process the request (or if the network interface itself is highly loaded) this will cause a delay in the response or basically cause packet loss. However, it should be noted that NICs can include a significant amount of their own processing power (not certain whether or not they can response to ping requests fully independent of the OS though??). Also in most cases the CPU will probably have some spare cycles as there's a good chance it will be waiting for some disk I/O to some extent.
There are also other things that might cause increased latency such as duplex mis-match between server/switch, or bad wiring.
All of that stuff aside, I'd be interested to know more about what made you ask this question in the first place? I assume that you have some kind of problem which you are attempting to troubleshoot via ping and would like to understand your results in more detail...?
Ping itself doesn't tell you much about what's going on with the server and why you may be experiencing some kind of slowdown/issue; for example if you website is taking a while to load, a common reason could be due to Apache and/or MySQL requiring optimisation to handle the amount of traffic better. In the case of Apache you could be waiting for a free connection slot to become available to answer you, or MySQL may just be processing queries slowly due to inappropriate cache/buffer config. etc. (which would slow down loading/generating database driven pages).
High amounts of disk I/O (for any reason, but one example could be thrashing caused by swapping) will obviously slow down any disk related operation such as reading files from your website to process a page, or loading up database tables etc.
It's difficult to determine the full reason for a problem like this without the ability to examine all of the details around it.
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