Considering the quantity of tags, I’d suspect you’ve simply exceeded the bandwidth of the PLC connections involved. The diagnostic pages of the PLC drivers will provide a great deal of information about this. You haven’t shared any driver information, so I have no specific advice. I can say that for some PLCs, driver advanced settings can make a big difference. Also consider making multiple simultaneous drivers for each PLC, splitting the tags in chunks between them. I would not expect splitting up the scan class to affect the performance, except possibly negatively in some pathological optimization cases. But adding leased scan classes can also help, by not having to scan some tags as often when your operators aren’t actively looking at them.
The “Tags in Ignition” section of Inductive University explains how all of this works in some detail.