这是套用 it is … that 构成的强调句,句中省略了连词 that.正常表达应该是 I can slow down the only time,意思是“我唯一的一次轻松起来（放慢了速度）”.句中用了表示时间的名词做状语.
有些带有the,this,that,next 等限定词时表示间、方式、处所等的名词可以直接用作状语.另如：
He will pay a visit to me (next week).
Come (this way),please.
这类名词还可以作为连词引导状语从句,如：
I'll report back (the moment) I have located him.我一找到他就马上汇报.
Sometimes things don't turn out (the way) we think they're going to.有时事情并不像我们认为的那样发展.