Downloading from nextcloud using Browser is very slow (~350KB/s)

EDIT: When I opened this topic I suspected a hardware issue which is why I’m listing all those details. However, I now know it has to be a software problem because I can get the expected speeds when I use the nextcloud client instead of downloading with a web-browser.

Nextcloud version: 20.0.4
Operating system and version: Arch Linux 5.8.9-2-ARCH (aarch64)
Apache or nginx version: nginx 1.18.0
PHP version: 7.4.10

Rock64 Single-Board-Computer with SD card for OS, 2x1TB HDD (btrfs-RADI1) connected via USB 3.
Nextcloud data and config are stored on HDDs.

The issue you are facing:
When downloading files from my nextcloud throughput caps out at around 350 KB/s. The uplink and hardware should be capable of much more. In the past I got around 6MB/s of thoughput from the same setup. I’ve investigated the I/O on the system and noticed something strange: during the downloads, I observe heavy write load on the SD card. It’s close to the amount of data read from the HDDs. These screenshot show statistics gathered with iostat in 2 60s intervals, i.e. I/O-load during the 2 minutes of the download. mmcblk0 is the SD card while nextcloud data resides on sdc and sdd.

This is my config:

$CONFIG = array (
  'instanceid' => 'xxx',
  'passwordsalt' => 'xxx',
  'secret' => 'xxx',
  'trusted_domains' => 
  array (
    0 => 'xxx',
    1 => 'xxx',
  'datadirectory' => '/path/on/btrfs/raid',
  'skeletondirectory' => '',
  'dbtype' => 'mysql',
  'version' => '20.0.4,
  'overwrite.cli.url' => 'xxx',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'xxx',
  'dbpassword' => 'xxx',
  'installed' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  'filelocking.enabled' => 'true',
  'redis' => 
  array (
    'host' => '/var/run/redis/redis.sock',
    'port' => 0,
    'timeout' => 0.0,
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,

I suspect something in my PHP setup but I have no experience with PHP , fastcgi, php-fpm etc. I’ve set fastcgi_cache_path to point to a directory on the same file system as my nextcloud data, i.e. the RAID1-HDDs but that hasn’t helped.
Can someone help me in narrowing down the cause?

EDIT: I’ve noticed that the issues occurs only when downloading via a web browser. When I sync using the nextcloud client I get the expected speed.