Upgraded to 500gb SSD, and still slow

So I bought a mid 2012 MacBook Pro, low price so I went for it.

It was super super slow the slowest thing I’ve ever used.

The mouse would lag around when you moved it. Safari would take minutes to just open. Any app was just unusable.

So I did some research and it seemed like I needed to upgrade to an SSD and more RAM. I’m new to MacBooks so that’s just what I thought I should do since everybody who has done the upgrade says there’s a night and day difference.

So I bought a 500 GB SSD and 16 GB of RAM and still no change.

I can’t even watch a YouTube video without it jumping every 2 seconds. I’m not sure if it’s the way I copied the OS to the SSD so tell me if I did something wrong .

I shutdown the system plugged in the SSD with a USB adapter and turned it on while holding command and R as I was told by YouTube. Then I went to “reinstall OS X” then I just clicked my SSD and let it download High Sierra.

And yes, I did erase the SSD in disk Utility before I downloaded it.

Again, I’m super new to upgrades and macs so I’d appreciate any help I can get to make my Mac usable and smooth thank you.