I just tried to fork your project and also checked the first person movement one from the tutorials. Both projects give me stable 60 FPS with and without transparent canvas setting. I also throtteled my CPU, and I get about 55 FPS with and without the transparent setting.
This setting should not affect the FPS. There must be something else, related to your browser.
@Ketan_ATA yes, I just tried it again. Cleared the cache to make sure.
Also, my laptop is from the same age as yours, but it is a PC using Windows/Chrome. Are you running in Safari? Can you test with another browser?
After checking Safari browser… I came to know that there is issue with Chrome.
Something is seriously wrong with my Chrome (Chrome Incognito mode also gives same issue) where game work smoother when Transparent Canvas is enabled.