Use Matrix or any good messenger like Signal or Threema for daily communication with friends.

If you want to see a good table of messenger recommendations, see https://www.messenger-matrix.de/messenger-matrix-en.html

E-Mail is not a suitable replacement because it lacks end-to-end encryption (unless you and your friends use PGP or S/MIME for that but since that’s rare and slightly too complicated for the common user to use, I’ll just assume that you don’t). While mails are usually encrypted during transport, they lie in plain text format at their destination servers. Depending on which e-mail host you or your friends use, that means the whole content of your e-mail is being scanned and analyzed automatically. Especially if you or your friends use privacy-disrespecting mail hosts like any big commercial one or Gmail or Outlook or what have you. Then your communication via unencrypted mail to or from that person isn’t private.

What do you want choose? An application? “Element” is a standard one and according to the webpage performance is a focus. I can’t say how well it works on your machine though. I don’t use Matrix anymore and the only way I used it was in the browser. So this list is more of pointing you in the right direction, rather than a personal recommendation.

• You can use Matrix in your browser: https://app.element.io/
• Or same software to install using it independent from your browser: https://matrix.org/ecosystem/clients/element/
• Alternative browser based client that should be light on resources: https://hydrogen.element.io/
• A list of clients: https://matrix.org/ecosystem/clients/
• CLI / Terminal based client written in blazingly fast Rust, if you really need a fast one and can live without a GUIf: https://github.com/8go/matrix-commander-rs