Unplug your Ethernet cable and see what happens.
I tried this. First I removed the laptop's cable just for fun, knowing AC would cease to function without connection to the server. Then I tried the better test of removing the practice's connection to the internet. I did this after AC was fully started up and I had logged in. I first tried eRX a few times, then disabled the internet, then tried again.
First few times I was seeing the normal 1-2 seconds for eRX. With the internet disabled it spent 10 seconds with the screen partly drawn, rectangle outlines visible, then another minute with the screen all white. I plugged in the internet back in.
This tells me conclusively that internet connectivity can affect how the eRX screen is drawn.
This is a really bad design because latency is going to be highly variable. Better would have been an interstitial "loading" screen of some kind, so blame could be placed on the right step of the process. Better still would be to have the screen load instantly with only local data, then update when internet connection was established.
This doesn't mean the internet is the only source of slowness for eRX, but just that it can cause eRX to run slowly.