The reason for Sept 1917 is the lack of a plane set - including no two seaters - for earlier periods. Once the plane set fills in the start date will go backwards.
About the flight leaders attacking ground units - sounds like I am going to need a bug fix release

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As for the time compression, that is internal to RoF. I am going to speculate here, but I think that as I add more units the RoF code is less able to handle it. It just can't do 8x compression with that many units.
Until I fix the flight leader wanting to attack ground units it might be best to turn off MGs (see RofCampaign.config - the useMGs parameter). That should also help compression as I think the little beggars are sucking up a lot of CPU.