I can only "see" about half of the computer VPN?

Were you running the IP scanner at the location with the missing PCs or at the other location? If the same location, then clearly not a Peplink issue. Windows networking hard enough on a single subnet. I gave up long ago and use nothing but IP addresses. Never names. Life is too short to fight with Windows all the time.

As for the slow router, this does not come as a surprise. The Balance 20 is “challenged” on firmware version 8. Search this Forum for more on that. For example

Perhaps fall back to firmware 7? Is there a feature in version 8 that you really care about?

Any VPN has to be a non-trivial cpu load on the router and the Balance 20 may not be up to the task. You didn’t say which hardware version of the B20 you have.