indicator-multiload (I/O wait)

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I´ve installed indicator-multiload in order to be able to monitor my network and cpu. Using indicator-multiload I noticed that the browsers (firefox as well as midori) cause a relatively high cpu-load.

This is when I open a new tab and contents are loading. Sometimes it happens that at least some reactions of the PC freeze for a few seconds. To be more precise: The actual cpu-load ("system") isn´t high at all. It´s rather what is called "I/O wait" that is really high (sometimes up to roundabout 70%). As soon as the respective page is fully loaded everything is back to normal again and the whole system is responsive like it was before. Of course this behaviour is dependent on the contents of the page. Just text or only few pictures mean no lag.

I want to add that I dont´t use any swap, neither as a separate partition nor as a swap-file. And: my system is installed on a USB-stick (64GB), USB 2.0 Yet I don´t know whether this has anything to with it.

My question is: Is there a way of manipulating the aforementioned behaviour, i.e. can I change in any way the cpu-behaviour with regard to "I/O wait"?

Many thanks in advance.
Greetings
Rosika

P.S.: system: Linux/Lubuntu 16.04.1 LTS, 64bit