After I installed on my Mac, my computer is definitely lagging, battery is running out way faster, and fans are louder.

From https://setiathome.berkeley.edu/info.php:

Is it safe to run SETI@home?

Any time you download a program through the Internet you are taking a chance: the program might have dangerous errors, or the download server might have been hacked. SETI@home has made efforts to minimize these risks. We have tested our applications carefully. Our servers are behind a firewall and are configured for high security. To ensure the integrity of program downloads, all executable files are digitally signed on a secure computer not connected to the Internet.

The applications run by SETI@home may cause some computers to overheat. If this happens, stop running SETI@home or use a utility program that limits CPU usage.

You can however set up BOINC to Suspend when computer is on battery (that's the default), Suspend when computer is in use and because you use the built-in GPU, Suspend GPU computing when computer is in use. The latter is also on by default.

So, when you see lagging, battery consumption being high etc. that's all down to misconfigured preferences. That the fans are louder is also normal, because you run intricate calculations under severe load on the central processing unit (CPU) and graphics processing unit (GPU).

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