Website optimization (the work of his scripts, pages) - the highest priority option. It is recommended to start with it.
This option includes not only fine-tuning the software included in the standard installation (Apache web server, MySQL, PHP), but also additional installation of various kinds of accelerators and changing the software configuration. Most popular options:
Another option, which is specially rendered separately - a complete rejection of the use of the Apache web server (slow, "heavy" in terms of resource consumption) and its replacement with a bundle nginx+PHP-FPM... It is very fast, economical, but requires serious enough skills to properly configure such a solution.
The last way to solve the problem is to increase the server capacity.
Provided that a server of adequate configuration was initially selected for the site, this is indeed the very last method. And it is worth resorting to it only in the most extreme cases, when all the others have exhausted themselves.
Unlike the previous methods, which require you to perform optimization work once (for money or on your own) and “forget” about the problem for a long time, buying a more powerful server is not even a solution to the problem, but self-deception. Elimination of symptoms, but not treatment of the disease. Sooner or later, site traffic will grow, there will be more content, and again you will have to spend even more money on a new, even faster server. Performing optimization actions will allow not only to reduce costs (even if you have to pay for the services of optimization specialists), but also to achieve better site performance than in the case of a simple increase in server capacity.