The PLC send out the Modbus command and wait for a Modbus response from the slave. If counts the number of data bytes received from the slave and when the correct number of response bytes have been received it is passed to the communication handler to verify the response. It does not wait for the additional delay of 3.5 character. The application program (TBASIC) can add the delay if necessary using the DELAY command between consecutive READMOBUS function.
What we found out is that in many applications if you only have single slave connected to the master, you can send consecutive MODBUS commands back-to-back without introducing delay. This allow maximum communication throughput.
When you have multiple slaves connected to the same Modbus master, you should add a small delay before switching the communication to a different slave ID. Usually if you don't change the ID then you can get maximum throughput without adding the delay. To maximize throughput you can do all the communication with the slave of the same ID, then add a small delay before switching to the next ID and so on.