Here's a quick benchmark with pgbench:

  $ pgbench -h localhost -p 5432 -b select-only -T 60 -c 10 -j 2 bench
  Password: 
  pgbench (15.3 (Ubuntu 15.3-1.pgdg22.04+1))
  starting vacuum...end.
  transaction type: <builtin: select only>
  scaling factor: 1
  query mode: simple
  number of clients: 10
  number of threads: 2
  maximum number of tries: 1
  duration: 60 s
  number of transactions actually processed: 10370147
  number of failed transactions: 0 (0.000%)
  latency average = 0.058 ms
  initial connection time = 54.881 ms
  tps = 172993.008029 (without initial connection time)

  $ pgbench -h /var/run/postgresql -p 5432 -b select-only -T 60 -c 10 -j 2 bench
  pgbench (15.3 (Ubuntu 15.3-1.pgdg22.04+1))
  starting vacuum...end.
  transaction type: <builtin: select only>
  scaling factor: 1
  query mode: simple
  number of clients: 10
  number of threads: 2
  maximum number of tries: 1
  duration: 60 s
  number of transactions actually processed: 16890415
  number of failed transactions: 0 (0.000%)
  latency average = 0.036 ms
  initial connection time = 7.186 ms
  tps = 281540.260418 (without initial connection time)

YMMV depending on your workload, but Unix sockets should always be significantly faster.