"磁带的起动时间" can be translated into English as "tape start-up time" or "tape activation time".
"Start-up time" is commonly used to describe the time it takes for a device or system to begin operating from a stationary or idle state.
"Activation time" can also be used, though it might be slightly less common in this specific context, as it generally refers to the time it takes for something to become active or operational.
In the context of magnetic tapes, "tape start-up time" is the more precise and commonly used term.