Does High Performance backend improve overall performance or just Talk?

I’d say either configure a password in Redis, or remove the line from the config.php. However I’d recommend to configure one, especially if Redis is exposed via a network port. Even better would be using a UNIX socket connection, if it’s running on the same server as Nextcloud.

And maybe a dumb question: But is the php-redis package/module installed?

Also, make sure you restart all releated service if you make changes to Redis config, or reboot your server.

That’s how I have configured it:

cp /etc/redis/redis.conf /etc/redis/redis.conf.bak
sed -i 's/port 6379/port 0/' /etc/redis/redis.conf
sed -i 's/\# unixsocket\ .*$/unixsocket \/var\/run\/redis\/redis-server.sock/' /etc/redis/redis.conf
sed -i 's/\#\ unixsocketperm\ .*$/unixsocketperm 770/g' /etc/redis/redis.conf
sed -i 's/# maxclients\ .*$/maxclients 10240/' /etc/redis/redis.conf
sed -i 's/# requirepass foobared/requirepass SUPERSECRETPASSWORD/' /etc/redis/redis.conf
usermod -aG redis www-data
cp /etc/sysctl.conf /etc/sysctl.conf.bak
sed -i '$avm.overcommit_memory = 1' /etc/sysctl.conf
systemctl enable redis-server
systemctl restart redis-server
  'redis' => 
  array(
    'host' => '/var/run/redis/redis-server.sock',
    'port' => 0,
    'dbindex' => 1,
    'timeout' => 0.5,
    'password' => 'SUPERSECRETPASSWORD',
  ),
  'memcache.distributed' => '\\OC\\Memcache\\Redis',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'memcache.local' => '\\OC\\Memcache\\APCu',

Source: Nextcloud Installationsanleitung - Carsten Rieger