Thank you for your answer, it took a while me to get back on this.
In Linux download: 1) I tested download of 10G test file from Nextcloud via Firefox browser. Download speed starts with 80 Mbit/s and drops to 5-6 Mbit/s. This download is in GE local network with HP enterprise switch and Watchguard firewall and no internet connection in between. 2) From Nextcloud directory in Nautilus (or what is the default file browser in Ubuntu), copy starts 106 Mbit/s and drops to 82 Mbit/s, but transfer is stable and works well.
In Linux upload: 1) From local folder to Nextcloud via Firefox, upload speed 120 B/s, so this is useless 2) Nautilus file browser, to Nextcloud folder from local folder copy over 100 Mbit/s, works well
Update 12-3-2021: I checked this tmp directory thing and enabled logging, no success.