Estimate safe use on public wifi.
Public Wi-Fi disappoints for two separate reasons, and this separates them. Throughput is a shared medium: the backhaul divides between active users, and because Wi-Fi is half-duplex with per-frame airtime overhead, aggregate capacity also falls as contention rises — roughly two percent per additional active client until it plateaus. A VPN then costs a further ten to fifteen percent to encapsulation overhead. Security is orthogonal. On an open or captive-portal network any device in range can see unencrypted traffic and DNS lookups, and a shared password provides only marginal protection because everyone has it. HTTPS covers page contents but still leaks which hosts you visit, which is what a VPN closes.
Shared hotspot throughput and exposure
Contention factor = 1 - 0.02 x (users - 1), floored at 0.35; fair share = backhaul / users x contention factor; usable = fair share x 0.85 when a VPN is active
The privacy rating here is a rough heuristic based on network type, not an assessment of any specific network or threat. It cannot detect rogue access points, captive-portal interception or compromised infrastructure. Avoid transmitting sensitive credentials or financial details over networks you do not control.
Usually not on a public hotspot, because you are already bandwidth-starved by sharing. Losing fifteen percent of a 7 Mbps share matters far less than having your traffic readable by strangers on the same network.
Only slightly safer than an open one. WPA2 personal derives keys from a passphrase everyone in the building has, so a party who knows it and captures your association can decrypt your traffic. Treat shared-password networks as public.
Distance and interference. Signal strength dictates the modulation rate, and a weak client drags airtime from everyone else. Moving closer to the access point often does more than anything else.