Arduino UNO and 8 channel relay turning on 2 relay at the same time via BT.

I just want to ask why is it slow? or delayed in turning on and off,

I have to press the button on the app many times before it responds.

Iv made the app in MIT app inventor. If you turn on 1 relay only its fast and responsive,

but 2 at the same time its slow.

Thank you