So here is an idea on assessing if players will lag before the game starts.
So the idea is to create a simple LuaSocket client and server. The host is the server and all other players (not observers) are the clients. Now there can be a few different ways to test the connection quality for all the players.
So here is a simple visual way, have clients move colour images from left to right (kind of like simulating unit movements).
So the colour block moves from left to right to left again. If the blocks move smoothly then there is no lag issue. If blocks are lagging then they can be give a score on how badly they are lagging by the colour guide on the left. If a player is showing RED then their connection has issues. More advance info can be added to show which players are lagging with who.
It may be possible to simulate all of this behind the scenes and just give a red block next to the players which are lagging, though something visual could be nice.