Modbus is not the best communication standard, only the cheapest. Most other comm protocols involve a licensing agreement and maybe a special chip set, Modbus is "Open Source"; kind of like freeware. Vendors provide it so that they appear to be compliant to industry's demand for fewer proprietary systems, but they do so almost begrudgingly.
A VFD has a somewhat complex microprocessor based control system that must make constant updates to the operating parameters of the motor firing circuit, so that always gets priority. Modbus communications, besides being relatively slow, usually requires processing power in the mP of the VFD to convert the commands to ts machine code, but it is not given very high interrupt priority. That is why most VFD manufacturers have their own protocol for communication between their VFDs and their PLCs; it is optimized to keep the overhead as low as possible. You have chosen to use a PLC from a vendor who does not make VFDs, and a VFD from a vendor who has no reason to optimize Modbus comms because it allows the use of other vendor's equipment. As long as you do that your comm rate will be less than optimal.
That said, one of the few VFDs on the market that does do a decent job of open communications is the Altivar 71 drive, because they went with a dual mP topology. One processor does nothing but the firing control, the other does all the other peripheral stuff such as communications, protection etc. But that is only the ATV71, none of the rest of the Schneider drives have that feature.
