Are still on chain gambling games able to mantain a decent performance, as often the mempool is clogged with high fees transactions?
On-chain gambling websites are usually using altcoins now since the usage on those are far less than on bitcoin's blockchain. When the Lightning Network gets adopted enough we'll see many more of those "on-chain" gambling websites. You'll be able to withdraw/deposit instantly on the LN. Till it happens, use altcoins.
When a coin becomes big enough, using on-chain for every shit transaction makes it unusable. We saw it with bitcoin and then Ethereum (remember crypto kitties)