which load can my wp bear?

Hello, thanks for supporting the optimization of my site. It is now optimized to grade A97 and faster than 97% of other sites, and loads in appr 0,5 seconds. After doing so, I doubled my server VPN capacity to 8Mb RAM and 4 CPU cores. However, it still looses performance at around 300-400 users, which which is hardly an improvement with 4Mb RAM, 2 cores. How can I analyze if some process is causing trouble, or is 8MB / 4 cores really not enough for a few hundred users?
Thanks