There's a moment
Let's say you needed to format dates in your SharePoint Framework solution. The moment you reference Moment.js in your project, the optimized Hello World bundle size will increase from 10KB to 278KB!
If you examine the bundle structure, you'll see that the bulk of its size is made of Moment.js.
Luckily, there is an alternative.
Instead of using Moment.js to format dates, you can use its light-weight alternative: date-fns. It has similar capabilities, but there is one big difference. Size.
When using date-fns in the same SharePoint Framework project, the bundle size will increase only to 37KB, which is 240KB saving comparing to when using Moment.js!
Give it a try and I'm looking forward to hear how it's worked for you.