Troubleshooting Slow Loading for One of Two Sites

Hello friends. I am running a WordPress Multi-site Network with two sites ans hosting on private server with Windows 2008 R2. One of the two sites is loading very slowly and I need help troubleshooting.

Here are some deets:

1. The themes of the two sites are different and neither have updates.
2. Plugins are activated on both sites, but not all for both sites
3. The Active Directory domain (domainDNS) is the nae of the first site and then the second site was added later. For example:

4. I have reduced the photos sizes on site 2 to improve loading time but it seems to have gotten slower – takes ~20 sec to load.

First site address is: site1.xxx.xxx
Second site address is: site2.site1.xxx.xxx.

I am not sure if all of these affect loading time but I wanted to give some info. Any ideas to get me started? Thanks in advance for your help!