Note that you can add metadata to the subscriptions that can later be used for filtering during dispatch.

Phoenix.PubSub.subscribe(YourApp.PubSub, "topic:user-id", metadata: [some: :extra_info])

which (if you are familiar with Registry) is similar to

Registry.subscribe(YourApp.PubSub, "topic:user-id", [some: :extra_info])

In general, the number of subscriptions doesn’t affect much, as the total throughput should stay about the same:

bench.exs

Mix.install([:benchee, :phoenix_pubsub])

{:ok, _pid} = Phoenix.PubSub.Supervisor.start_link(name: App.PubSub)

defmodule Sub do
  use GenServer

  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, opts)
  end

  @impl true
  def init(opts) do
    topic = Keyword.fetch!(opts, :topic)
    Phoenix.PubSub.subscribe(App.PubSub, topic)
    {:ok, nil}
  end

  @impl true
  def handle_info(_message, state) do
    {:noreply, state}
  end
end

Enum.each(1..200_000, fn _ -> Sub.start_link(topic: "topic") end)
Enum.each(1..10, fn _ -> Sub.start_link(topic: "topic2") end)

map = %{"some" => "key", "then" => "some oter key"}

Benchee.run(
  %{
    "message" => fn topic -> Phoenix.PubSub.broadcast(App.PubSub, topic, "message") end,
    "map" => fn topic -> Phoenix.PubSub.broadcast(App.PubSub, topic, map) end
  },
  inputs: %{"lots subs" => "topic", "few subs" => "topic2"}
)

> elixir bench.exs

Operating System: macOS
CPU Information: Apple M1
Number of Available Cores: 8
Available memory: 8 GB
Elixir 1.13.1
Erlang 24.2

Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 0 ns
parallel: 1
inputs: few subs, lots subs
Estimated total run time: 28 s

Benchmarking map with input few subs...
Benchmarking map with input lots subs...
Benchmarking message with input few subs...
Benchmarking message with input lots subs...

##### With input few subs #####
Name              ips        average  deviation         median         99th %
map          147.35 K        6.79 μs  ±1531.35%        4.99 μs       24.99 μs
message       95.00 K       10.53 μs  ±9207.44%        2.99 μs        8.99 μs

Comparison:
map          147.35 K
message       95.00 K - 1.55x slower +3.74 μs

##### With input lots subs #####
Name              ips        average  deviation         median         99th %
map              3.12      320.16 ms     ±9.22%      313.18 ms      387.97 ms
message          2.78      360.19 ms    ±16.91%      352.94 ms      479.31 ms

Comparison:
map              3.12
message          2.78 - 1.13x slower +40.02 ms