It’s probably not a good idea to pull ALL tags into your HMI project as this just adds to the overhead. However, the issue isn’t really on the gateway side per say, but rather on the PLC side of things. We have systems with much higher tag counts, and ran into issues with the PLC CPU being overloaded, as well as showing in Ignition as it being overloaded.
What we ended up doing was adjusting the period rate on the PLC from 20ms (I think this is default) to something like 100ms, and this gives the CPU more time to process communications traffic (5x roughly). That might be enough for you to reduce the load seen by Ignition to about 50%. Based on the Mean Response Time (99.1ms), I’m guessing you have a periodic rate set to 100ms. To get it under 100%, you will have to bump it to 300ms to give you some margin. You might also have to adjust the Request Packet Interval setting as well, but also look into the System Overhead Time Slice, as increasing this will allow more throughput.
Some of this is discussed in this thread.