Possibly the best you can do is to use the library presented here:
Extended modbus library for Siemens PLC's.
There is ease of use and some optimization has been done in the processes.

Otherwise the process design Siemens makes available to us is linked to the scan of the PLC which does not run well enough in a polled process, like other brands have available. The process itself is slow in execution.

The use of Profinet or Profibus is the best to do when you have several devices. Else jump ship and try another brand's equipment.

Hello,

yes, agree with qwazee. Modbus RTU is simply quite old protocol and also very slow for large number of devices, especially in case that more steps are required to do with each device, then logically complete process is longer. All depends to your project, nothing special. In case you have still free some from CPU left positions (now you have only one CM?) you can easy cut your MB bus to more independed 'networks' and have better processing of data with more Slaves, but still is not so quick. Generally, speed can be set to higher in case. As qwazee already mentioned, in case of more devices are expected and speed plays big role of data processing update, more expected protocols like Profinet (with its versions) or Profibus are required.

To your question:

Am I right? Would it be like this: I press a button, and then only after 4 seconds pump will start?

probably yes but I expect this procedure like typical pre-process preset options and parameters set and only after correct check that all is read/set correctly, you can allow to expected person to push/click Start. I don't see nothing bad with this procedure, maybe wait for that, correct data need to be set for correct process functionality, correct?