In case of you have performance issues on Linux/Unix machine definitely using one of these tools (or combined them).

Ready carefully this article: http://www.tecmint.com/command-line-tools-to-monitor-linux-performance/

Presented tools: Top, VmStat, Lsof, Tcpdump, Netstat, Htop, Iotop, Iostat, IPTraf, Psacct or Acct, NetHogs, iftop, Monitorix, Monit, Arpwatch, Suricata, VnStat and Nagios