Standard SQL is rubbish for time series queries as it is based on set theory which does not have order. Most SQL datbases exploit that fact to increase performance. Fundamentally kdb is based on ordered lists, which is a much better paradigm for time series data.
Have you checked questdb [1]? the data structure is arrays with data that lands in order and SQL queries on top. The fallback was that it was difficult to deal with out of order data, but we have just solved this by re-ordering data on the fly in memory before it hits the disk. Performance wise probably not far from kdb itself (will be sharing some bench results soon vs open source tsdbs)
The contrasting queries can be seen here: http://www.timestored.com/b/kdb-qsql-query-vs-sql/
I do agree they could use competition as the overall offering is weak but the core database is very strong.