Then I go to check the Activity Monitor and sometimes I can't see anything unusual.
Sometimes it lags even when watching YouTube video or playing a movie.
Even after restart, I have to wait for the kernel_task to calm down, because sometimes it goes up to 1000%. Sometimes it works smoothly, sometimes it lags when I edit photos in Lightroom Classic or Photoshop, and sometimes Final Cut is completely unusable to the point that I cannot do anything but either leave it for who-knows-how-long, or Force Quit and restart the machine. I have no apps in 'Login items', but I have 1Password, Adobe Creative Cloud and Backblaze booting up with the system, which I have for many years. I recently noticed that my 2019 MacBook Pro (2.4gHz, 64GB DDR4, 8TB) started lagging even if no apps are opened.