Speed up Nextcloud

Not uploading
Not uploading more than a few MB at a time
Not uploading more than a few files at a time
Not downloading
Downloading/Uploading slowly

Mess with these files:

Your php is probably messing with most of that. Find out where you php is located and also which version you have.
Min is located here
/etc/php/8.1/fpm/php.ini 

Edit this file. Inside this here you should find a few fields:

upload_max_filesize = 16G
max_file_uploads = 3000
post_max_size = 16G  
memory_limit = 512M

Edit them to a higher limit.

Restart php and server nginx/apache2

sudo systemctl restart php8.1-fpm.service
sudo systemctl restart apache2.service # For Apache
sudo systemctl restart nginx.service # For Nginx