@markus7017 I tested a bit more.
Good news: There is no single timeout message in the openHAB log any more. 
regardless how much and how quickly I switch the shelly 2.5 with firmware 1.5.9 - everything switches as quickly as the shelly can (in roller shutter mode).
The debug channels Timeouts and TimoutsRecovered are all the time during switching at zero ‘0’.
But I was not able to get Wireshark capturing the packets. I am not experienced in using Wireshark. In my setup I may would have to do more network re-config than I can/want to do to capture these packets. Therefore I would looking forward for someone else to do such captures.
@markus7017 would you be able to create me a private build of the binding that is similar stable to the release 2.5.1, but with the fix? Or could I use the current one until you have released an official fix in an upcoming openHAB release?