Adding the -cpucount=4 parameter to ARMA II does utilize your 4 cores and should improve performance.. As for number 2, it could be the server you're playing on and/or one of the beta updates having a negative effect on the games performance.. It's not unheard of.
Would make somewhat more sense, i know that when servers start becoming full that really impacts how the game runs. Just got me a little bit weirded out, guild wars 2 runs at a low ish frame rate anyhow dependant on where i am, at the moment in the start of the asura race i can drop from 30 - 16 fps dependant on veiw and situation. But im all out of ideas on how to increase performance now.