nsl.exe is a process belonging to NetStat Live.
It monitors the TCP/IP protocol which displays the through-put on both incoming and outgoing data. This is a non-essential process.
Disabling or enabling it is down to user preference.
Non-system processes like nsl.exe originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that over time your registry suffers fragmentation and accumulates invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues