Well, we finally appear to have resolved this issue in our environment. For the benefits of others, here's what we discovered and how we fixed the problem:
To try and gain further insight into what was occurring before/during/after the delays we used Wireshark on a client machine to capture/analyse network traffic whilst that client attempted to access a DFS share.
These captures showed something strange: whenever the delay occurred, in between the DFS request being sent from the client to a DC, and the referral to a DFS root server coming back from the DC to the client, the DC was sending out several broadcast name lookups to the network.
Firstly, the DC would broadcast a NetBIOS lookup for DOMAIN (where DOMAIN is our pre-Windows 2000 Active Directory domain name). A few seconds later, it would broadcast a LLMNR lookup for DOMAIN. This would be followed by yet another broadcast NetBios lookup for DOMAIN. After these three lookups had been broadcast (and I assume timed out) the DC would finally respond to the client with a (correct) referral to a DFS root server.
These broadcast name lookups for DOMAIN were only being sent when the long delay opening a DFS share occurred, and we could clearly see from the Wireshark capture that the DC wasn't returning a referral to a DFS root server until all three lookups been sent (and ~7 seconds passed). So, these broadcast name lookups were pretty obviously the cause of our delays.
Now that we knew what the problem was, we started trying to figure out why these broadcast name lookups were occurring. After a bit more Googling and some trial-and-error, we found our answer: we hadn't set the DfsDnsConfig registry key on our domain controllers to 1, as is required when using DFS in a DNS-only environment.
When we originally setup DFS in our enviroment we did read the various articles about how to configure DFS for a DNS-only environment (e.g. Microsoft KB244380 and others) and were aware of this registry key, but had misintepreted the instructions on when/how to use it.
The DFSDnsConfig registry key must be
added to each server that will
participate in the DFS namespace for
all computers to understand fully
We thought this meant that the registry key has to be set on the DFS namespace servers only, not realising that it was also required on the domain controllers. After we set DfsDnsConfig to 1 on our domain controllers (and restarted the "DFS Namespace" service), the problem vanished.
Obviously we're happy with this outcome, but I would add that I'm still not 100% convinced that this is our only problem - I wonder if adding DfsDnsConfig=1 to our DCs has only worked around the problem, rather than solving it. I can't figure out why the DCs would be trying to lookup DOMAIN (the domain name itself, rather than a server in the domain) during the DFS referral process, even in a non-DNS-only environment, and I also know I haven't set DfsDnsConfig=1 on domain controllers in other (admittedly much smaller / simpler) DNS-only environments and haven't had the same issue. Still, we've solved our problem so we are happy.
I hope this is helpful to the others who are experiencing a similar issue - and thanks again to those that offered suggestions along the way.
In light of your most recent update and comments, I think that your best bet is to allow connections to
Server Avia VPN access directly. It doesn't make a lot of sense to involve Server B in the mix at all.
There are packages in the Samba 4 release that can potentially do SMB proxing, but I haven't heard of anyone using them with any level of success. You certainly can't do this with any native Windows tool.
In light of your updates, this isn't an answer to your question, but it's still good knowledge nonetheless, so I'll leave it.
There are two different DFS technologies: Namespaces and Replication.
A DFS Namespace allows for multiple file-servers to have the same UNC. For example
\\domain\sharecould be backed by
\\server2\share. The users have no idea, they just connect to
\\domain\shareand are transparently redirected to one of the backing file servers.
You can define what server users are connected to by a number of ways. One of the most common is by what AD Site they are in. If you want your users to access the share on either server transparently by
\\domain\share, then you want to use DFS Namespaces, but this isn't a complete answer to your problem.
DFS-R allows for files to replicated (go figure) across multiple servers. If you had
\\server2\sharein a 2-way replication group, then any changes to either share will propagate to the other. If you want your two servers to have the same contents, then you will use this. It can be used independently of a DFS Namespace, but many times it is used in conjunction with it for seamless access to resources based on AD Site or for redundancy/loadbalancing in general.
You're not "resharing" like your question asks how to do, but rather you're keeping an actual copy of everything in both places, including permissions. This is, at a minimum, what you need to accomplish what you're asking.
If you choose to use DFS Namespaces in addition to DFR-R, then that's up to you and is a design decision. It will certainly simplify access from your users, but is not completely necessary.
tl;dr Use DFS-R and maybe DFS Namespaces, but definitely DFS-R.
Edit: Since it seems that my text wasn't understood clearly, here's a nice picture