@Mark_Harwood, @DavidTurner - first of all, thank for you all your hints here 
finally some good news here
Got it fixed - I have disabled filterEditor:suggestValues in Kibana settings so KQL won't suggest values in discovery search and problem is gone.
filterEditor:suggestValues enabled:
|Query time|271ms|
|Request time|53354ms|
filterEditor:suggestValues disabled:
|Query time|380ms|
|Request time|1718ms|
Finally it all works as it should, question is - shouldn't this feature get reported as a buggy? As I read in other threads it looks up all indicies instead of those specified in time range picker which causes such CPU load.
Anyway - I hope that this workaround will help others having same issue with ELK 7.