Web application has frontend and backend and "some features loading more long" may be caused by either slow rendering or slow backend response.
First of all go for your browser developer tools and see where the page spends the most time. At this stage you can also use frontend performance testing tools like Lighthouse
This way you will be able to state whether it is a frontend or a backend issue.
If you have an APM tool in place you can inspect the slow call and trace it down to the underlying problem (slow database call or heavy function, or waiting for a response from an external application or slow network or whatever).
When it comes to "classic" load testing tools - none of them can "tell" you what is the problem, they can only mimic hundreds/thousands of users so you could measure how does your application perform under the load, where the problems start occurring, whether application recovers when the load decreases, etc. There is no "best" tool, there are different tools built in different technologies, some have GUI, some require programming, some can be run in clustered mode, some are standalone only, some support only 1 network protocol, some are multiprotocol. In order to come up with a piece of advice we need to know your requirements (and maybe skills), check out Open Source Load Testing Tools: Which One Should You Use? article for example investigation and comparison.