MyBB Community Forums

Pro Stats plugin is terrible to run and some ad providers are crap too and will cause heavy site slowdown.

Thanks To All
For Co Operation..

list your plugins, I may give the suggestion

Mybb Support Team is No1 in the World,
I am Proud On Mybb & Mybb Support Team
here is the list of plugins installed in my forum,

Add Spiders (0.1)
Ads after first post (2.2)
Advanced Quick Reply Form (1.0.3)
Ajax PM Notification (1.8.1)
Akismet (1.2.1)
Auto Media (1.1.7)
ChangUonDyU - Extra File Chatbox (3.6.0)
Contact form (0.1)
Default Avatar (1.0)
Fit on Page (2.3)
Google Analytics (1.5)
Google SEO (1.1.13)
Hello World! (1.0)
A sample plugin that prints hello world and prepends the content of each
Iframe BBcode (1.0beta)
Login Password Conversion (1.0)
Megaupload On MyBB (1.0)
Miss You Email (1.0.0)
My Question (1.3)
Guests See No Threads (1.0)
Show the users that has been online today (2.0)
Page Manager (1.5)
PM Spy (1.0)
PM On Registration (1.2.0)
PM On Reputation (1.0)
PostAd(lite) (1.0.0)
ProStats (1.7.5)
Quick Register (1.0)
MyBB Quran (1.0)
Show Users Who Have Been Registered Today (1.0)
Reputation Power (1.0)
RSS Feed Poster (1.0.4.1)
Thanks (3.9)
Tinypic On MyBB (1.0)
Undo Delete (1.2.1)
View Unread Posts (1.3)
User Agent Reporter (1.0)
Username Style (1.0.4)
yAnnnouncement (1.0)

Chatboxes are known to slow down load, and I'll also say that an iFrame MyCode is likely going to be opening your site up to various exploits.

Right,
is there any maintenance task to do in Mybb..

A lot of bad plugins. I'm not surprised your site is slow. I'd ditch 1/2 of those.