I understand the feeling. I was pretty disappointed with the Key2 when I played with it a bit at an event: it's an OK phone, but coming from the Priv with its gorgeous OLED screen, the Key2 is such a huge downgrade.
I had exactly the same problem as you did: my Priv was unbearably slow at common tasks, so as much as I've loved the phone, I got frustrated with it almost daily.
Nevertheless, I'll share that although I'm pretty happy with my Note 8, the keyboard is definitely a big step down from the Priv, so be ready/set your expectations for that. E.g. everything else will be better than the Priv, minus the keyboard, which will be measurably worse.
It's possible we'll have another landscape slider soon: https://www.readyfx.com/ so I'm also keeping an eye out for that.
I don't expect the note keyboard to be as good as the Priv's, and it hurts enought to give up the shortcuts and the capacitive actions. However, the Priv is near unusable - google maps takes over two minutes to load sometimes. I want a brand name phone that has a camera at least as good as the Priv's, with a reasonable (doesn't have to be stellar) physical keyboard. I bought the Key2 and returned it after a day - the screen was just awful and the camera even worse.
I'm sure I can adjust to the Samsung keyboard and it's certainly much better than typing on screen (something I was never willing to do besides a year with a nexus 4).