Your network load is not the problem. Based on your JSON export, most of your tags are contiguous, words 22-35, with three others: 6-7, 44-45, and 66-67. Ideally, you need four requests to read these blocks, satisfying the ten float tags. Per the modbus specification (PI-MBUS-300 from modbus.org) these requests and their responses should take 62.5ms at 19200-N-8-1 (including the inter-message silent time on the wire). Twenty devices is a modest overload–you won’t get one-second timing.
I believe Ignition will make the requests in the pattern above if the “Span Gaps” setting is turned off in the driver. I suspect that your devices have registers (other than your ten) that respond poorly, and spanning gaps will read unexpected registers. Spanning from word 6 to word 67 would take 75ms, but would read up to 31 floats, which is why it is default “on”. Try turning that setting off.