蒋老师解释的非常好。下面为COCA语料库类似例句：

Can you explain how this round differs in the speed it travels as compared to a.40 caliber round? 

Given the volume of information and the speed it travels, the job has become vital to achieving the Pentagons military objectives.

其实，先行词为speed时，可以用关系副词that引导定语从句，that在从句中作状语。以下为COCA语料库关系副词that引导定语从句的例句：

And it allows you when you are moving to document the speed that you are traveling.

In each case it happens at the speed that it would have happened in real life and ends as abruptly.

Especially with the speed that I'm swinging my driver, I feel the shaft in the driver can get a little weak.

 Oddly, the speed that something is going doesn't really affect how deeply it digs into the ground.

BNC语料库关系副词that引导定语从句的例句：

So we're talking about seveal thousands, if not getting closer to millions, of the speed that a human being would possibly travel?

Never forget that it is not the speed that the horse jumps the fence, but the rhythm maintained and picked up immediately on landing and the economy of route both into and away from fences that gives a fast time.

Alcoholic Intake # Is the speed that people sip/guzzle beer or other intoxicating liquor and therefore the speed they get pissed directly proportional to their desire/need to press on in their resolving of life's problems?

以上例句说明，在英国英语中和美国英语中，关系副词that都可以引导定语从句修饰speed，that在从句中作方式状语。而关系副词that总是用于非正式语体，所以that常可以省略。所以，网友的上述例句其实也可以理解为省略了关系副词that，而不是省略了介词短语at which.