High end VPS is Slow

I have bought a vps to host my website. I have installed apache2, php, mysql server. My vps specs are:

ram: 30 gb guaranteed

cpu: 8 cores

ssd: 800 gb

port: 600 Mbps

os: Ubuntu 18.04
i have about 500 active users who are watching videos on my android app using my VPS using bandwidth around 200Mbps. Now the problem i m getting is that if I have about 200 users everything working fine response time is great on my app video start immediately but when user reach about 400 video dont even start for a minute what could be the problem.

my ram usage is about 1 gb buff/catche sometime increase to 20gb i don’t know how my apache don’t even have a cache module.

my apache config is

 <IfModule mpm_prefork_module>
 StartServers       5
 MinSpareServers    5
 MaxSpareServers    70
 MaxRequestWorker         1000
 MaxConnectionsPerChild   0
 </IfModule>

I really can’t understand, what could be the problem. Can anyone help me?

ps. Sorry for my bad English.