|If you have a LAN (NIC) connector which I am sure you do you will have been made a member of a workgroup (usually MSHOME) by default.|
So first the answer to your question is you do not normally turn this off.
I suspect that file and printer sharing has been enabled on your laptop so that your laptop can see other PCs on your network and vice versa. Along with that some of your files / directories may have been shared so that they are visible in your network.
First it is unlikely that your laptop will have slowed down because of this.
There may be other related reasons though which are difficult to ascertain.
You can if you want turn off file and printer sharing. You might not want to as you may see a benefit in maintaining file and printer sharing as it allows you to network your PCs in your home network.
To turn off file and printer sharing:
Start > Control Panel > Network and Internet Connections > Network Connections
Right click on Local Area Connection and select Properties
Uncheck the box that says "File and Printer Sharing ..." and Click OK
If the above is unclear follw this:
☺ When everything else fails, read the instructions.