On my Apache Server Status, there's no idle servers left:

21.7 requests/sec - 0.9 MB/second - 41.7 kB/request
201 requests currently being processed, 0 idle servers

The download speed from the server's still fast, but accessing the web pages (http) are very slow. Is this because there's no idle servers left?

Should I increase the MaxSpareServers value in httpd.conf? The current value is 15.

Please see this thread for more info:
http://www.webhostingtalk.com/showthread.php?threadid=117924

Try changing a couple of things to see if they help:

Timeout 30
#initial value should be around 300

KeepAlive Off
#initial value should be On

MinSpareServers 15
#initial value should be 5

MaxSpareServers 20
#initial value should be 10

HostnameLookups Off
#initial value should be Off

Once you make any of the above changes you will need to stop apache (make sure there's no processes left running) and then start the server up again. If you find that its still slow, you may want to see how logs are being generated as well.

Thanks for the tips. I'll definitely try it out!

BTW, do you know what's the Linux command to kill all the hanging or zombie processes at once?

If there is any, I may run it every 15 minutes as cronjob :)