300 isn't problem problem if ping is stabile that is why i posted diagram of sa:mp client to show you how pings jumps and makes me laggy.Also Vert you are on end of world and ping of 400 is normal for you i am less then 2000km away from server so ping should be less,around 70 in best case.
Well all problems solved only ping is bit high around 100 not ISP fault cause they buying internet speed from Telekom (main ISP in Serbia hosted in Belgrade) and some problem exist on UP link,should be fixed soon