That situation is explained in the faqs:
https://wordpress.org/plugins/fast-velocity-minify/faq/
FVM needs to generate the css and js files once, on the first pageview, on every page that has different requirements (different sets of css/js files, or different inlined js/css code).
This occurs once, on the first pageview, however, it also depends on your site.
If you are using some plugin or theme that keeps changing the css or js file in every pageview, FVM will detect it as a new requirement and thus regenerate it again. When merging all files, even if only one character changes, the whole cache file needs to be created.
The process of creating that initial cache file, is resource intensive, but it only happens once (usually), however some plugins have dynamic classes and dynamic code, and even dynamic css or js filenames where they append the time of each pageview (this is bad practice), so that means, FVM keeps detecting different files and different code.
If in addition, your server is slow and if you have a lot of traffic, it will cause 100% cpu usage simply because, the server is busy generating that cache file, while more people are browsing other pages and doing the same.
This can be seen if you see many different files on the status page (there should be only a few of them usually), especially if they show the same url on the log. It also makes your cache grow forever, with every pageview.
Do note, FVM is a plugin for developers and advanced users.
A developer or advanced user, will be able to see which code keeps changing on every pageview and be able to exclude it via FVM settings.
In addition, you also need to use some sort of page cache plugin if your hosting doesn’t already have such cache functionality.
If you use a cache plugin, even if your plugins are generating dynamic content, it will be cached for a while and this means, FVM will also not trigger the process again.
Cached pages, are not supposed to trigger PHP, therefore FVM will also not have any work to do.
It’s some plugin you are using, that is either creating that dynamic situation, or you have too much traffic and your server is to weak to handle the optimization process.
Also note, if your pages are too big, the html minification library can take some time on a weak server. That’s simply how the server works and there is nothing to do other than to disable html minification or reduce your content in one page, to a normal size.
An article with the top 100 things to do all in one page, it’s definitely going to take a long time to minify and use up 100% cpu (one core per pageview) while it’s doing it.
Minification is heavy, that’s why you need to exclude any dynamic code and file names, so that minification only occurs once.
I suggest not trying to do speed optimization if you cannot see what to exclude, and instead hire someone to do it for you.
It may work for the majority of sites, but in some cases it doesn’t and it needs someone that understands what’s going on.
