Just got into Arma 3 and back in to pc gaming.

I've been looking at mods. I've never modded a PC game before. How do mods affect playing online? Are there servers that don't allow mods? Will mods slow down performance (running at ultra now)?

I want to get some mods that make the game more current timeframe, uniforms and weapons and such, as well as blast core (I think that was what it was called)

My understanding at least from ARMA2 was that if you tried to join a server with a mod activated that wasn't in the server's mod list, it wouldn't let you join.  Same goes for if you try joining and don't have the mods required on that server.

For example, if you use the (excellent) JSRS sound mod, you can only take advantage of it on servers where the JSRS key has been added, otherwise it will kick you.