Regardless of what SSID people are using if you get to many people on your guest network that is still going to affect your other SSID because capacity is shared amongst the entire access point irrespective of what SSID users are connected to.
Now you could enable traffic shaping on the network and that might help to a degree.
If you found this post helpful, please give it Kudos. If my answer solves your problem, please click Accept as Solution so others can benefit from it.