Slow browsing experience is a behavior that can happen for so many reasons that cover everything in one single article is just not feasible, mainly when the list still growing. Here are some posts/articles that I wrote on this matter:
The list is growing because recently, troubleshooting another issue of this nature today I learned another cool thing: disabled adapters matters. What I mean with this is that if you have multiple adapters on your ISA/TMG and if some of the adapters are not in use and are disabled, this can still affect the performance of the system. How? According to the binding order. Remember that I talked about binding order here some time ago? The DNS best practice analyzer pointed me to the right direction on this, here what he said about disabled adapters on the top of the binding order:
Valid network interfaces should precede invalid interfaces in the binding order. A disabled or invalid adapter precedes a valid adapter in the network interface binding order list.
The binding order determines when network interfaces will be used to make network connections by the computer. A disabled adapter high in the binding order can degrade performance.
Move all disabled and invalid interfaces to the bottom of the binding order list.
Keep this caveat in mind during your ISA/TMG performance health check analyzes.