If you don’t enable Optimize Prebuild Only (which is recommended), then all visitors, who are visiting an uncached page will start a new thread to optimize the actual page.
If you have 20-30 or more visitors at the same time, it can easily overload your server. To avoid it, you should limit simultaneous threads.
It is also recommended to limit threads if you set Prebuild Speed to Multithread.
If your site is running on Shared hosting it is recommended to set it to 3-4. On very limited shared hosting you should start with 1.
If your site is running on VPS or dedicated server, it is recommended to set Limit Simultaneous Threads to lower value than the number of processor cores x 2
For example, if your server has 8 cores then the maximum recommended value of Limit Simultaneous Threads to:
8 x 2 -2 = 14