CI3 - count_all_results() is slow

Im using count_all_results() in CI3 to create the max results for pagination.
My main query that has the limits set and returns the full result is very quick and optimised / indexed etc
The issue is that I also run a version of the main query with count_all_results() instead of get() and this is often pretty slow.
Is there a better way to do this or am i just missing a trick ?
All my tables use InnoDB on MariaDB 10.3
Thanks in advance for your help