How will mining work as this is an ERC20 token if I'm not mistaken?
PKT is ERC20 token, you are right. Mining in Playkey is a little bit different: instead of calculating hash, miner shares his powerful computers, servers with gamers and receive PKT as a payment.
don't you think Ethereum smart contracts is too slow for a game mechanics ?
Playkey will temporary use offchain solution to speed up transactions inside the blockchain. PKT is only for payment between users and service providers, and doesn't affect gaming experience at all.
How users will can buy this tokens? May they buy tokens for fiat on your platform?