nab agent.exe is part of the SymantecAntiBotAgent by Symantec. This prevents a PC from getting hijacked through Active Behavioral Analysis.
Non-system processes like nab agent.exe originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that your registry has suffered fragmentation and accumulated invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues.