Can't repro? Could be specific router ddos prevention stuff.

It is reproducible for me - just not 100% of the time (but quite high degree).

This seems also a fairly common issue going by other people's reports in BI forums and on Steam.
It is most certainly connected somehow to the user hardware, router, firewalls, or ISP.
Just found no advice yet anywhere what can be done about it and Arma 3 launcher has no configuration unlike Steam (AFAIK).

Working as intended:

Not working:

as seen its usually dead slow with only few servers a second - aka to scan all 6k takes ages

ingame its always fast. so i can only suspect the way the launcher establishes the connection to Steam can be the problem
like some anti ddos measure gets triggered or some rate limit

i have tons of mods subscribed. so maybe steam finds this request suspicious and downgrades the MP query then

https://discordapp.com/channels/@me/228801001723789312/727909855003934770
i tried various the various max pings/minute in steam, but none has any visible affect in the Arma 3 launcher (side from the expected slowdonw ingame). ingame its never an issue

I've had this issue for quite some time, happens every time with me as in the first 'Not Working' video, at first i thought it was a router port conflict with other computers on my network, but even with those powered down and disconnected the issue is still there.

My router and switches have been on the same firmware's for quite some time, I haven't done any software changes to them and no cable routing has changed.
For what it's worth my entire network is running at 1GbE on Cat7 cabling and have no cable issues, cable tester reports all lines are good.

My computers are all running Windows 10 and are fully up to date as MS allows (ATM I have one PC on Win 10 1909 and the others on 2004).

Steam is set to Automatic(5000) - Even manually changing this value from Automatic to 5000, 3000, 1500, etc makes no difference to the launcher.

same issue here since like forever