I have personally tested on 4-5 USFF PC's ranging from intel J1900, J4115 and i3 CPU's. All used Realtek network hardware and all were affected. All were initially using the R8169 driver. Many Other Linuxcnc users have reported the issue. All of these had hardware covered by Realteks official R8168 driver. All of these benefited from installing the Debian R8168-dkms driver.
Compiling the RT kernel is not new to linuxcnc users as it was required up until Debian first released linux-image-rt packages. All we have ever needed to do was to patch the code and make a single change in menuconfig/xconfig to select the fully preemptible kernel and compile. I learnt how to build kernel debs when Bookworm was on the 6.0 kernel and built a 6.1-rt5 version which I shared publicly with other users via Google Drive. This resolved issues for a lot of users. Another user recently reported substantial improvement in latency with the 6.3 kernel so two of us built and tested it with outstanding and near identical results for both overall latency and network latency.
I have not kept my .config files as PC's have been reformatted so many times. However, my kernels and the steps used to build them are available in my google drive. They will show what we have changed. Here is the link to the 6.1.0-rt5 kernel
https://drive.google.com/drive/folders/1jGc6AUYKMPvsSOdWRdvhWeDX1P96tsFQ?usp=sharing I will redo this for the final 6.1 kernel and share the config to get in step with Debian Bookworm's current state.
Note we use Linuxcnc's latency testing tools to measure latency but cyclictest produces similar observable results.
Unfortunately, we don't have any portable method to test network latency. Our hardware reports the maximum time to read and write to it in CPU timer ticks.
This command may give some insight but the other device would need to be on a dedicated point to point ethernet connection (no hub or router)
sudo chrt 99 ping -i .001 -q 10.10.10.10
I hope this covers your questions.