We are aware of this double-posting / "30 second warning" issue and it is related to our ongoing efforts to optimise the performance of the server, especially during the peak times of 10pm - 1am.
		
		
	 
 
I think this tweaking doesn't helps, should revert back to original setting, this double posting/30sec warning had become more of a nuisance (to the admins and mods especially due to the numerous double/triple or even more duplicate posts) then helping on the server performance i believe (why i say so is because users will definitely feel frustrated and keep clicking on the post button when they see this 30sec warning, that will in fact create more load to your database as duplicate posts chunk out much or a while later).
can try this guide:
http://www.itsabhik.com/server-optimization/vbulletin-server-optimization-tips-part-1.html 
Optimal vBulletin Options
    * Enable All AJAX features – Make sure that this is set to allow all AJAX Features. Ajax helps reduce server load by not reloading a page where AJAX can be used. (Quick Reply for instance) > General Settings -> Disable AJAX Features and select Enable All Ajax Features.
    * Cookies and HTTP Header Options > Remove Redirection Message Pages – I personally have not disabled it for Go4Expert.com but for my client I had to put this off to save on an unneeded page load and bandwidth.
    * Cookies and HTTP Header Options > Add No-Cache HTTP Headers – Make sure you don’t opt for No-Cache setting and so set this to No.
    * Server Settings and Optimization Options > Cached Posts Lifespan – If you have no issues with space on your server setting this to higher value is recommended. I have this set to 10 days and once you turn this on rebuild your Post Cache under Maintenance > Update Counters
    * Server Settings and Optimization Options > Update Thread Views Immediately – Set this to No. This will update thread views via a cron every 1 hour by default. If you wish to change this you can do so by editing the Scheduled Task for Thread Views to a different time.
    * Server Settings and Optimization Options > Update Attachment Views Immediately – Set to No.
    * User Listing & Profile Viewing > Show Last Post on Profile Page – This is very server intensive process and so set this to No.
    * Message Posting and Editing Options > Quick Reply – Not only does this make posting faster which leads to more posts being made this also uses Ajax which means you do not have to load another page to make a post.
    * Message Posting and Editing Options > Quick Edit – Turn this on to reduce the need for a new page reload when someone simply wants to edit a post.
    * Message Searching Options > Minimum Time Between Searches – Set this to some value mainly to avoid an attempt to use the search to harm your server.
    * Forums Home Page Options > Display Logged in Users? – I have mine set to Yes but if you have issues with server load you can turn this feature off.
    * Forum Display Options > Show Users Browsing Forums – I have mine set to No and if you have issues with server load you can turn this feature off.
    * Thread Display Options > Show Users Browsing Threads – I have mine set to No for similar reasons as above.
    * Forum Display Options > Maximum Displayed Threads Before Page Split – Having too many threads on a single page makes the page load slowly and so ideally you should have the value 20 or 40. I have this set to 20.
source - 
http://imtips.co/optimize-vbulletin.html
if u have access to the server: 
http://www.howtoforge.com/using-mem...letin-forum-to-reduce-server-load-debian-etch (
Vbulletin also recommends using memcached)
I done some of these before in the past when i still owns a vbulletin 4 based forum.
Disclaimer: This post is not telling what you guys should do. Just sharing what I had done in the past (I had experience using Vbulletin since version 2 till 4). Just in case my intention is being read wrongly by you guys.