A way to disable the SMB signing mechanism at the Apple Time Capsule?
Okay, here follows a litte addition to my original question. This is not a final answer. It can be seen more as the first part of a &quot;real&quot; solution. ;-)
As mentioned in the comment section by user Gumminando, Apple is using at the Time Capsule a (modified?) Samba Server. According to the official doc the SMB1 &quot;server signing&quot; feature can be defined in the smb.conf file as follow:
(Source: https://www.samba.org/samba/docs/current/man-html/smb.conf.5.html)

server signing = default
server signing = auto
server signing = mandatory
server signing = disabled

Apple is using at their TC models most likely the mandatory parameter which enforces the SMB1 signing in any case.
It should be noted that all these options are only SMB1 related. The SMB2 signing (of newer Samba versions) cannot be disabled because it seems to be &quot;in design&quot; mandatory. Well, a SMB2 functionality is not present at any Apple Time Capsule model so that aspect can be ignored anyway.
So what would be now the final solution? In theory, you just have to log / hack into the TC and modify the smb.conf file with parameter server signing = disabled or server signing = auto. Unfortunately, it's not that easy in practice.
However, I can confirm that the slow SMB1 signing affects primary larger files. I have now tested this for a while and the overall behavior with smaller files like office documents is okay. Well, this may be true in our case because the original TC hard-disk drive was replaced with a Crucial MX500 2TB SSD. Yes, this works GREAT.
And finally, the idea to use also under Linux the AFP protocol to access the Time Capsule seems interesting. Unfortunately the current AFP Linux implementation is somewhat broken (and incomplete). The SMB1 implementation may be slow but it is definitely very stable and reliable. I would describe it as rock solid.