SSL VPN/MACos/MTU issue?

Hi all

Running a sophos XGS with a PPOE connection. Firmware is the latest 19 incl MR. MTU is set to 1500 (which I understand is minus 8 meaning 1492).

On this we have an SSL VPN terminating on the XGS using TCP. Clients are Mac OS running tunnelblick and windows running sophos connect.

Purpose of VPN is to access SMB shares on Synology NAS.

On a Mac..for some reason the dragging and dropping of files from the NAS shared folders onto a word doc (in order to add the file to the doc as an image) causes the machine to lag as the file is dragged over, right at the point it leaves the finder window.

On a pc it's flawless and smooth so I'm trying to figure out how to make smb smoother on the macs.

I've tried.

Changing the Mac OS setting to speed up browsing and not hammer the DS store file that gets made by Mac.

When on VPN pinging the Nas the largest MTU I can use is 1372 (process of ping elimination). I haven't set the MTU for the VPN anywhere I can see. MTU on the Nas is a standard 1500 and the switches in use LAN side are dumb switches along with 2 ports on the sophos bridged to the LAN zone/side.

Checking CPU usage on client and firewall. Nothing is stressed as we're talking the bottom end XGS for 3 users parallel max.

Swapped to an Asus router using openVPN that comes built in on it and also the client for Mac and PC and this issue was not present Pensive hence my journey down the rabbit hole to fix this.

Any help appreciated, and If there's any specific config I can post from CLI or GUI please do let me know. Im not an expert by any means.

Edited TAGs
[edited by: emmosophos at 4:57 PM (GMT -7) on 23 Sep 2022]