In my experience, single largest impact with web sites will come from carefully adjusting apache and database settings.
Apache/PHP should be adjusted to maximize RAM usage and be optimized to expected number of users/connections on the system.
PHP might be configured in different ways too, and there are caching tools/modules for PHP too.
Same thing for database, mariadb is a complex beast and adjusting it properly is not an easy task, but upside is great. There are good scripts for monitoring mariadb performance with suggestions on how and what to do. I am using tuning-primer.sh and mysqltuner.pl.
I am runing suiteCRM on my own (enterprise class) server, so I have as much resources as I wish, but situation on some virtual hosts or shared metal might be significantly different.
Implementing REDIS is also an option (it does not come by default) and I see there were discussions on using and configuring REDIS with suiteCRM: