Alright, that's a problem I've been working at in my server.
Lemme tell you some steps, first make an ACL account and include it in admins group.
Second, you've to use a script called performancebrowser in MTA:SA default scripts folder.
Third, open this link in your browser "http://yourserverip:22005/performancebrowser/"
For ex. "http://12.105.123.43:22005/performancebrowser/"
Then, it will ask you for the ACL username you've created, write it then you will see the page.
Tick "include clients", then you will see players names appeared.
Choose the player the faces the FPS Drop and see what resources make high usage in CPU.
Try to figure out why it does that and I can help you if you can't fix the script yourself.