Question: I am using OSX 10.7.5 and my OSX is becoming a bit unresponsive, comparing to what it was like before. Sometimes just opening new window of browser or terminal takes few seconds or the Dock hovering effect is not fluent. At these moments I watch system resources and I still have 3G of memory free and the processor is not used at all. Can it be caused by hard drive or other IO? How can I found out which application is causing this?

Answer: 
Use `top` to monitor system resources.
As you mention issues with opening the browser possibly there is malware active or some other agent which is causing unwarranted network activity. To monitor this use-:

```
ifconfig -a  # for the network interface name to monitor

```

then

```
tcpdump -i <interface>

```

An alternative for network monitoring is `nettop` - a command line tool which I believe is available from Lion version. It's available from Apple store.
Check the logs for clues. Even a high volume of relatively harmless error messages (itself an error!), could swamp your response time.

Edit: /var/log/system.log and /var/log/kernel.log would be most likely. Or open the Console app in Utilities and show all messages.