Hmmm…that’s not how zwave is supposed to work according to the independent third party protocol: you can have devices at any of the different security levels and they should all be able to handle normal network traffic without bypassing anything with two exceptions:
- you can only set up direct association between devices at the same security level
- Devices with the highest security level, normally door locks, may not accept commands from some devices at a lower security level, typically handheld remotes.
But other than that, any mains powered light switch, for example, should be happy to pass along a message from any sensor regardless of their relative security levels. That’s part of the “backwards compatibility“ that is built into the standard. Here’s a statement from the company that now owns zwave and designs the security specifications:
Z-Wave never compromises on backwards compatibility. We want existing devices to interoperate smoothly with new controllers. If you come across a 10-year-old Z?Wave device, you will find that it includes and operates seamlessly with even the newest controllers on the market. We believe that the chosen security solution follows that tradition and strikes the best possible balance between security and interoperability. We have a clear path forward for evolving security across the entire ecosystem to become even more robust without sacrificing convenience and interoperability.
So what you are seeing is not supposed to happen just because you have different zwave devices at different security levels.
That said, I have no idea what happens in the SmartThings implementation of the S2 standard, the cloud architecture introduces some idiosyncrasies. 
If smartthings engineering verifies that there’s an issue if you have devices at different zwave security levels, I believe them, but then they should be publicizing that much more widely since it’s a very significant deviation from the standard.
Hopefully someone who is more familiar with the smartthings details will respond. Quite a few people have been reporting over the last couple of weeks that they are seeing significant Z wave slowdown, but yours is the first time I’ve seen any explanation given.