At our main site we have 2 Domain Controllers running 2008R2 . We had a satelite site that untill last week didnt have a DC ,our tech in the office created a DC and joined it to our domain, since then we have noticed a big AD 'Slow down' so im assuming its because in AD we only have one site. I know i need to create another site and the subnets to go with it. Is this something that could be done during 'work' hours or will it lead to certain amount of discruption so it should be done out of work hours?
You can perform this work on the fly without any negative impact in the Active Directory Sites and Services console...
Be sure to define your Subnets first, then the Sites and associate the domain controllers with their respective Sites.
FYI - if you want to influence users/computers not in a site (such as VPN users) to not use the DC in the satellite office, you may use what are known as "DNS Mnemonics" to modify the DNS record registration for the satellite DC. You can read more about that here:
How to optimize the location of a domain controller or global catalog that resides outside of a client's site
You can either edit the registry on the satellite DC, or use the use the Net Logon service Group Policy "DC locator DNS records not registered by the DCs".