Does changing startup disk do anything?

My 2012 MacBook Pro has become pretty slow recently so I considered changing HDD to SSD. However, I have a 1TB My Passport external hard drive. Is it possible to change the startup disk to the external one and if I can do that, will it allow my Mac to run faster?
Please don't be sarcastic if this is a stupid question, I'm honestly not sure.
Thanks

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