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Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 21.0.1
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04.2 LTS
Apache or nginx version (eg, Apache 2.4.25): 2.4.41
PHP version (eg, 7.4): 7.4
The issue you are facing:
When connected to my NC instance locally, I cannot seem to get above 40MB/s download speeds without a CPU core being completely pegged at 100%. 40MB/s is not terribly slow by any means, but I know my setup can achieve much higher I/O.
Is this the first time you’ve seen this error? (Y/N): Y
Steps to replicate it:
When using the web client, if i try to download a file of around 2GB my speed is capped around 40MB/s. If i download multiple files at the same time, I can achieve total throughput of around 60MB/s but each file being downloaded completely maxes out a CPU core.
The output of your Nextcloud log in Admin > Logging:
No Errors.
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => ‘*******’,
'passwordsalt' => ‘*******’,
'secret' => ‘*********’,
'trusted_proxies' => ‘*******,
'trusted_domains' =>
array (
0 => ‘*******’,
1 => ‘*******’,
),
'datadirectory' => '/nextcloud-data',
'dbtype' => 'mysql',
'version' => '21.0.1.1',
'overwrite.cli.url' => ‘*******’,
'dbname' => 'nextclouddb',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => ‘********’,
'dbpassword' => ‘*******’,
'installed' => true,
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_smtpauth' => 1,
'mail_smtpname' => ‘*******’,
'mail_smtppassword' => ‘******’,
'allow_local_remote_servers' => true,
'mail_smtpsecure' => 'ssl',
'mail_from_address' => ‘******’,
'mail_domain' => ‘*******’,
'mail_smtpauthtype' => 'LOGIN',
'mail_smtphost' => ‘*******’,
'mail_smtpport' => '465',
'maintenance' => false,
'enable_previews' => true,
'enabledPreviewProviders' =>
array (
0 => 'OC\\Preview\\PNG',
1 => 'OC\\Preview\\Image',
2 => 'OC\\Preview\\TIFF',
3 => 'OC\\Preview\\MP3',
4 => 'OC\\Preview\\Movie',
5 => 'OC\\Preview\\MKV',
6 => 'OC\\Preview\\MP4',
7 => 'OC\\Preview\\AVI',
8 => 'OC\\Preview\\BMP',
9 => 'OC\\Preview\\HEIC',
10 => 'OC\\Preview\\GIF',
11 => 'OC\\Preview\\JPEG',
12 => 'OC\\Preview\\3GP',
),
'htaccess.RewriteBase' => '/',
'default_phone_region' => 'US',
'theme' => '',
'loglevel' => 0,
'memcache.local' => '\OC\Memcache\APCu',
'updater.release.channel' => 'stable',
'twofactor_enforced' => 'false',
'twofactor_enforced_groups' =>
array (
),
'twofactor_enforced_excluded_groups' =>
array (
),
);
The output of your Apache/nginx/system log in /var/log/____:
No Errors.
