How can I find out why my forum is running so slowly?

[محروم.كوم]

My forum is using vBulletin 3.8.6 Patch Level 1 and in recent times it has been running very slowly - sometimes it takes ages for pages to load and it is very uncomfortable to use.

As these stats show it is not a very big or active forum;

• Threads: 7,740, Posts: 56,183, Members: 19,310, Active Members: 567
• Users active in the past 48 hours: 1603 (86 members and 1517 guests) - this is pretty typical and we don't experience any huge spikes in activity.

I know the problem is not my PC or my ISP, because other sites are loading fine for me, SpeedTest.net shows that my Internet connection is fine and the slowness has been reported to me by some of our members.

Hosting - the forum is on a VPS from a very reliable hosting company. The VPS has the following capacity;

75 GB Storage
1.5 TB Monthly Transfer

1.5 GB Guaranteed (3 GB Burst RAM)

I'm not an expert but in my opinion that package is much larger than the forum should require - given it's size and activity levels. I reported the problem to the host just over a week ago and they have been looking at it but still haven't come up with a definitive reason as to why the site is slow. So far this month the forum has used "15.28 Gig" of data transfer. There are other sites on the VPS but they are all very small and all combined they use about the same amount of bandwidth as the forum.

How can I find out what is causing this problem?

Thanks! __DEFINE_LIKE_SHARE__