How important is it to get service soon after a warning that "battery capacity is significantly reduced"?

Very important. It needs to be checked out at Genius Bar.

Non functioning battery can affect performance.

Keep monitoring the health of the battery.

"Service Recommended" is the notification to watch for.

About batteries:

https://support.apple.com/en-us/HT204054

TomHart1 wrote:

How important is it to get service soon after a warning that "battery capacity is significantly reduced"?

Learn how to get your Mac fixed and how much it will cost. https://support.apple.com/mac/repair/service

Get your Mac ready for service - Apple Support https://support.apple.com/en-us/HT209095