Hello @hieutp0203
I am sorry for the inconvenience caused, actually, the loading speed of the page is depends on many aspects but usually this happens when there is a over usage of the plugins on the website.
The reason behind that the many plugins that you will you will make your website heavy because each and every plugin will utilize a server resources to execute their scripts.
As, many resources are being utilized, the server will take a but more time to free it’s resources and process new request and response.
To find out what is affecting the speed of the checkout page, you can try deactivating all the plugins except the WooCoommerce, CartFlows & Page Builder and then check the speed of the checkout page. I am sure that you will get a improved speed.
Now, activate the plugins one-by-one and check the speed of the checkout page simultaneously. I am sure that you find one or more plugins which are taking more resources and making a website bulky.
I hope this helps.
Feel free to get in touch in case you need more help or assistance.