Howard, I am sharing information in hopes that this issue can be resolved wherever the problem is which is unknown. I will give you further information if it helps.
Work computer(s) are PC based and use Firefox and the ISP is a DSL through Verizon.
Home computer(s) use Safari and are newer, Macbook Pros and the ISP is Xfinity (Comcast) cable.
Both locations have the same problem, both locations are in different cities. I am using 2 different computer systems, Mac & PC, 2 different browsers and 2 different ISPs with the same results. Loading this page to see your post took almost 5 full minutes. Again, I cleared both my cache and cookies to all sites and also tried Firefox from my Mac with again, the same result.
I do not have this problem with other sites that I frequent, others around the country have expressed issues with the speed of this site so maybe it is a propagation issue? I have no idea. What I do know is that the problem is legitimate and needs to be resolved in one way or another.
I feel your pain because I too run a Vbulletin based forum and had this same complaint and could not understand why everything looked good on our end. In the end, I moved to a single server in a new location and have never heard a peep from anyone since.
I know this is quite a large forum so I recently went to a much larger forum that is also Vbulletin based and have no issues there. As a matter of fact it is probably a bit faster than my own. Offtopic.com is a very large forum that uses Vbulletin and has over 145,731,638 posts and well over 235,000 registered members. Whatever they are doing is working.
A few years ago, a local company started getting complaints about access to their website and it was specific to a few ISPs. They ended up moving servers because there was a connection problem between their new server and a few ISPs. Not sure how you are set up but there is an issue at some level that is legit. I hope this helps in a a constructive way.