I’m still new on this casino and still exploring. There some time that when I play house game with multiple time an error message shows asking me to slow down.
I had experienced the same issue when I was playing the Plinko game by opening 3 different windows. There was no issue like that as long as I was playing the games by opening two windows only. I consider it as a normal thing. You will also face some lags in other casino if you play the games by opening multiple windows at the same time.
I’m late to notice the competition existence when I create my account. Is it possible to participate even without using BCTalk ref code when I register?
Unfortunately, it isn't possible if 24 hours have passed already after your account creation.

You are allowed to enter the affiliate code manually within the first 24 hours of the account creation. In that case, you have to write the code in the 'enter referral code' section on your account page.