Indexing seems much faster if low_memory=false but it makes indexing suspend and not sure how to push it further.

In log I can see:
INFO: Mon Mar 20 13:10:35 GMT 2017: codeIndexLinesCount 1148817 larger then 1000000
INFO: Mon Mar 20 13:10:32 GMT 2017: Pausing parser.

There is no such case with low_memory=true

Also with low_memory=true CPU usage is even 25% all the time (one of 4 cores I guess), but with low_memory=false it stops using CPU at all when the buffer hits limit.