Originally Posted by Stratocaster

Um, it's so hard to understand because it's wrong. No 2 rides are the same. There's ALWAYS a varying amount of speed left on the table, regardless of whether you're on an 18 lb bike or a 20 lb bike. Sometimes you will leave more on the table with a lighter bike. It's just what happens.
In a math problem, when you can say "all things being equal"...then yes, you are correct. The lighter bike is always faster.
In practice, with real humans...the differences can disappear in a hurry.

Heck, while riding the lighter bike you might think to yourself, "I know this bike is lighter than my old bike...I can feel it. I'm just cruising along on this thing!"
Yeah, you might be "cruising along" and so pleased with how you feel about it, that your intensity drops.
There are more components to the discussion than just weight of the bike.

But yes - in a math problem where "all other things being equal" - the lighter bike is always faster.