New M1 Mac - connecting to another Mac very slow / broken.

I have a new MBPro14 which I've set it up from scratch. No Time Machine.

It has a problem when being connected to from another Mac and which is then browsing its folders/files. Browsing them is agonisingly slow, with the spinning wheel becoming the spinning beachball and network speeds dropping to zero and finally, just nothing happening. The only way to escape the beachball, is to turn off the wifi/unplug ethernet.

Browsing in the opposite direction is fine and fast (From MBPro14 to the other Mac).

Connecting via Screen-Sharing is fine and fast in both directions.

I've tested this with 4 other Macs:

• over normal internet-box wifi

• over an airport Extreme AC, not connected to the internet... so purely between 2 computers

• over old fashioned ethernet cable, plugged in to 2 computers

The other Macs are:

MBPro13 M1 running Ventura

2015-MBPro running Mojave

2013-MBAir running Monterey

2008-MBPro running El Capitan.

The results are identical in every situation... Fast file browsing of other Macs via the new M1 MBPro 14 ... agonisingly slow and eventually stalling completely from the other Mac browsing the M1 MBPro 14.

Sometimes connecting the the MBPro14, simply shows nothing. So it's connected, but no folders appear in the Finder window.

All Macs have identical username (Admin) and identical password and are connected via User. (I've always done this without problems and I assume they all have their own UIDs). 

There are no issues between all the other Macs, browsing each other. So it's not a networking issue.

I've tried everything I can think of re: smb/pref deleting/User permissions.

Browsing the MBPro13 M1, via any of the other Macs, is blisteringly fast. Sigh...

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Before I go through the pain of a re-install of the OS, to eliminate that as a problem, which I guess would only leave a hardware problem... can anyone suggest something that might fix it?