A few days of stress testing showed what can be done. Using Rustdesk it is possible to transfer files at "full speed" from my two remote computers simultaneously to my local home computer.
"Full speed" here takes into account various limitations, which in my case are the following:

Remote broadband service
500/500 mbps allows fast file transfers, several at the same time.

Local broadband service
250/50 mbps can handle at least two simultaneous downloads.

Remote network card
10/100 mbps limits the speed to a maximum of 100 mbps (don't forget to make sure the card is set to 100 mbps). Going with a gigabits card in one computer raises the upload speed to 500 mbps (or 60 Mb/s), but limited to 250 mbps (30 Mb/s) by my home service provider.

Local network card
Gigabyte network card, so the bottleneck at home is now the local ISP.

Network cables
Avoid wifi, cable is required if it is to go fast. The network cable must also handle the desired speed, so Cat 6a is a good choice.

Computer internal bus
Nothing to worry about when it comes to new computers, older ones usually have a bus speed of 133 MB/s, which can create some delay.

Hard disk write speed
With 100-200 mbits, most hard drives can keep up, but with higher speeds a fast SSD is preferable. In the case of several simultaneous downloads, as in my case, there is some queuing as the disks do not have time to receive all the data fast enough. It is solved by downloading to two different hard drives and preferably not a USB disk.
If only a single download is running at 100 mbits, it doesn't matter what kind of disk is used, traditional hard disk, SSD or USB disk, the disks are fast enough.