C. Fast or slow update?

Before we answer this question, let's take a short look on how top really works. Strace is your friend here:

$ strace -o /tmp/trace.txt top -b -n 1

Use you favourite text editor and load /tmp/trace.txt. What do you think? A lot of jobs for single invocation, that is what I think and maybe you'll agree. One of the jobs top must do in every iteration is opening many files and parsing their contents, as shown by the number:

$ grep open\( /tmp/hasil.txt | wc -l

Just for illustration, in my Linux system, it yields 304. Closer look reveals that top iterates inside /proc directory to gather processes information. /proc itself is pseudo filesystem, meaning it doesn't exist on real disk but is created on the fly by the Linux kernel and live on RAM. Within directory such as /proc/2097 (2097 is a PID), Linux kernel exports information about related process and this is where top gathers processes information along with resource usage.

Also try these:

$ time top -b -n 1

This will give you illustration how fast top works on single round. In my system, this yields around 0.5-0.6 seconds. Look at "real" field, not the "user" or "system" field because "real" reflects the total time top needs to work.

So, realizing this fact, it will be wise to use moderate update interval. Browsing RAM based filesystem takes time too, so be wise. As rule of thumb, 1 to 3 seconds interval is enough for most users. Use -d in command line option or press "s" inside interactive mode to set it. You can use fractional number as interval, e.g: 2.5, 4.1 and so on

When we should faster than 1 seconds?

One way to reduce top's work is by monitoring certain PIDs only. This way, top won't need to traverse all the /proc sub-directory. How about user name filtering? It won't do any good. User name filtering brings extra work for top, thus combining it with very short interval will increase CPU load.

Of course, whenever you need to force the update, just press Space and top will refresh the statistic right away.