@DDS90 The “Modbus polling stopping” is completely unrelated. This discussed more in Modbus Binding not working on OH3
Tl;dr it’s a regression in the serial library used by openhab.
I cannot download the log files (keeps on “connecting”), perhaps you could just grep the timing info?
Trace level logging indeed introduced additional cpu burden 
you can try setting trace level logging only for the relevant logger with the following command
log:set TRACE org.openhab.core.io.transport.modbus internal.ModbusManagerImpl
With some effort, you might be able to configure logging to output only the timing info (regex filter combined with trace level logging of one particular logger)
The timing info should confirm what you already seem to know… I.e. How much time goes into read/write activity with the slave and how long poller things are “waiting for their turn” to communicate with the device.