There’s nothing faster than a horse in Chinese, right? Saying you’ll do something “on a horse” doesn’t mean you’ll be wearing Old Spice while you do it, but that you’ll do it immediately. There is something faster, though – fire.
Here’s the usual words you’d use to express immediacy:
Wǒ mǎshàng bāng nǐ zuò
I’ll help you right away
If you want to express even more speed than horseback, then you can say ‘at the speed of fire’:
Here’s an example of 火速 from a Mandarin Sherlock Holmes book I was looking through today:
Jǐngchá jiē dào bàoàn hòu, huǒsù gǎn dào xiànchǎng
After receiving the report, the police officer hurried to the scene at top speed
Even though 光速（guāng sù）”the speed of light” might be faster than 火速, you can’t use this in its place, except maybe jokingly.