It would not slow the watch down.

When activated on your iPhone's service plan, the built-in cellular capability on Apple Watch Series 3 (GPS + Cellular) enables certain features to remain available when using your Apple Watch away from the paired iPhone, when they would not be available on other models.

The functions that are available on your Apple Watch when it is away from your iPhone depend on various factors including:

• Your Apple Watch model:
  • Apple Watch - Compare - Apple
• The features of your carrier service plan:
  • Apple Watch - Carriers - Apple
  • Make a call with Wi-Fi Calling - Apple Support
• The type of connectivity that is available to your Apple Watch at the time:
  • About Bluetooth, Wi-Fi, and cellular on your Apple Watch - Apple Support

Features and requirements that apply under various circumstances are detailed here, including within the footnotes:

• How to use your Apple Watch without your iPhone nearby - Apple Support