We have experience with a few sites running 500-1200 online members. Some adjustments were needed and have been added to UNA 10 to improve performance, but nothing in the architecture prevents UNA from handling larger loads.
I’m trying to only rely on actual experience, but as soon as we have sites with millions of members to work with, we will adapt the system. It’s not so much a matter of current capability, but a matter of core architecture and readiness to improve.
If it is a server issue, I offer both VPS and Dedicated servers and I can transfer your site from UNA servers to my servers for free. If this is an UNA issue, I have worked with UNA but I am not as versed in UNA as I am in Dolphin.