If by performance you mean “will it slow down throughput or increase latency compared to the same wifi without a portal system” then I would say both should be unnoticeable in terms of performance, with a few caveats. The task you are basically asking the portal system to do is apply one ACL to users that haven’t authenticated, and a different ACL to users that have. This may involve traffic to internal resources (printers, servers, etc). Also, the portal needs to be presented.
With PFSense, the application of the ACL is done at the router, versus at the AP for Unifi. This difference has two effects. The first is that PFSense could theoretically become a bottleneck if you had a fast enough internet connection with a slow enough router, whereas AFAIK every Unifi AP has a fast enough CPU to match the amount of traffic it can do on its wireless. The other effect is that if you want un-authenticated users to not have access to servers and printers, they have to be in a separate subnet and VLAN with PFSense, otherwise the traffic to them would not go through the router. With Unifi since the ACL is applied at the AP, in theory the wireless could be the same subnet/VLAN (I’ve never tried this and I still wouldn’t recommend it). This difference also means that the PFSense router has to be fast enough to apply the ACL for internal traffic also. But this is a fairly simple issue to not worry about - just get a router that can handle a gigabit internet connection, since that also means it can handle gigabit on the LAN side.
A PFSense router can store the portal page directly on it, and it is very fast to present the files to users. Unifi instead has the AP that a user is connected to proxy the user to the controller, which then serves the file from its storage. Unifi’s way is pretty fast too, but I imagine that it would take slightly longer (worst case a few seconds) to display than PFSense.
So basically, each system has its good sides and bad sides when talking about whether it slows things down, and whether it affects the layout of the network.