Looking through the items on my WiFi - most seem to be on 192.168.39.xxx
Most are on that IP address range??? What are the others on?
See this is why people are asking...there is something messed up in your set-up that is causing your issue. Here is another way of the diagram with the IP address of everything on it labeled taken from another thread - it is diagrams like this that usually clear up the problem. As you can see, the proper set-up should be your cameras on one IP address range and everything else on a different IP address range. If yours are all the same, they are going through the router (even if not reaching the internet) and is taxing the CPU and thus the reduction in internet speed.
Whether you want to believe it or not, taking the time to draw this out and label each IP address of EVERYTHING on your system will probably make it clear to you what the problem is, and if not, it will be clear to someone here. Guessing and saying "most are" is not going to solve the problem.
And putting your cam IP addresses do not tell anyone anything - they are the same as everyone else - again you do not understand enough to help people solve your problem. The IP address of your service provider for your WAN is what you don't provide...Everything on the inside past the modem is fine to put out. Everything on the inside, the local will fall under these ranges and you are not telling anyone anything about how to hack your system providing these ranges (basically any IP that starts out 10. or 172. or 192. are reserved for the "home side" of the service so every home internally will be within this same range):
10.0.0.0 – 10.255.255.255
172.16.0.0 – 172.31.255.255
192.168.0.0 – 192.168.255.255
I get it - I was there too once. Look it up for yourself outside of this forum on these internal address ranges and see for yourself that providing these causes no security issues.