Vaadin 8.3.3 Grid DataProvider Query limit

I have an UI with an Grid and get strange values in my DataProvider for the requested limit (and offset) in the Query.
Sometimes limit is 40 (DataCommunicator.minPushSize?) sometime limit is 25 somtimes 105, …
The limit value seems not to be related to the count of visible rows in the UI!?
Sometimes I even get a second fetch request with an offset, for rows not visible in the UI.
Who can explain this stange behaviour of Grid while fetching rows to display in the UI?
Could this be the reason for the poor performance of Grid?

Thx in advance