There are several free and paid WordPress plugins for live chat. WP Live Chat Support is a free live chat plugin with more than 50,000 active installations.
However, it can affect site performance. I installed it on my blog but had to remove it because the site pages loaded slowly due to this plugin.
If you are looking for a lightweight WordPress live chat plugin, you may want to consider Live Chat Unlimited.
It's lightweight.. This plugin works in iframe not to hurt your page performance. The widget loads itself once your page fully loaded.
After the one off payment for the Live Chat Unlimited plugin, there are no other costs such as monthly fees or other pricing plans making it a great choice for those who just want an online chat solution with no additional costs.
With this plugin, 100 users can chat at one time and you can install it on unlimited domains.
It' also offers multi-lingual support (WPML and Polylang plugins.) And it's possible to localize all UI strings of the Live Chat widget into your language.
It's possible to show offline form, when you're away.
To enable real-time chat, you need to create a new Firebase project and generate a Private key, which is well explained under the Real-time tab.
For more information, visit the Live Chat Unlimited page.