There are various reasons why CSF firewall may make your WHM / server slow.
Here are some personal tips from me.

1.Change your default SSH Port from 22 to something else. It will prevent (although temporarily) malicious bots from scanning your server, and adding extra load to your firewall.

2. Edit your CSF firewall configuration, and change these configuration :

a. CC_LOOKUPS : 0

This will disable IP country lookup. Imagine your firewall blocking hundreds of IP each second, and doing additional country lookup for each IP.

b. LF_EMAIL_ALERT : OFF

c. LT_EMAIL_ALERT : OFF

d. CT_EMAIL_ALERT : OFF

e. PS_EMAIL_ALERT : OFF

*_EMAIL_ALERT things will disable sending email from your server to your e-mail address which will add extra work for your server if your server is scanned by hundreds of IP each second.

If you have other tips for improving CSF speed, please comment below .

• Source : xrvel