If you have problems between 2 clients, then you need to troubleshoot from an end-to-end perspective.
You can have issues on your local side, on the remote end or a mix of both and of course in the WAN / internet.
Can you please describe how the PCs are connected to the internet router on your and at the remote end ?
Are you using LAN or WLAN technologies to connect to the internet ?
How is the Internet Bandwidth (Down-/ Upstream) and technology being used on your and the remote end ?
Do you use cellphone based services like i.e. cell based (LTE, ..) where the medium is being shared between
devices in your radio cell ?
To exclude local PC and LAN/WLAN problems I would suggest that you connect 2 PCs via cross-cable to check
whether with a normal Ethernet any issue shows up.
Next step would be to repeat this test using the Switch on your local Internet Router.
Don't use Wireless services, its not recommended for this software as the author tells.
You need to perform all these checks on your and the remote end.
Always ensure in the LAN, that you use 1 Gbps "Full Duplex" connections, Half-Duplex is a no-go.
For Gigabit ports its best to have both ends to use Auto/Auto settings for speed and duplex.
With half-duplex connections you will get collisions on the shared LAN media.
Your question on their forum seems to be here: http://forum.sessionlinkpro.de/viewtopic.php?id=716
I found another interesting Forum article: http://forum.sessionlinkpro.de/viewtopi … d=491#p491
They recommend to test with this latency tester https://test.webrtc.org/?test_filter=Network%20latency
In another thread I saw a posting about a guy having problems with this software, because he had Malware on his MAC, did you scan/check for malware on your or your friends PC ? You can check this with i.e. a product from malwarebytes.
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