Wireless keyboard/mouse adaptor may be a cause of a FE issue (there might be others I suppose).
Removing it (at Wolfram's suggestion - credit for investigation so far) made MMA10 startup speed effectively normal & no "formatting notebook" popups were noted. Unfortunately I don't have a physical keyboard & mouse...
[Setup i3, Win-7 pro 64-bit; all Acer hardware].
Additional info: running kernel directly showed no autocomplete lag on entering test expressions for evaluation, suggesting the issue is related to the FE.
See also my post at Wolfram community