The bug

When banners with multiple patterns are rendered, the frame rate impact is higher than banners with no layers at all. This can be seen when placing a certain number of banners with a single color (in the creative menu) against a number of banners with +6 layers (more layers = worse frame rate).

White banner (no layers):

Blue banner (8 layers):

Blue banner (16 layers):

This issue may be minor as players aren't able to craft banners with more than 6 layers, but there are certain banners that can only be obtained with commands, which might be an issue in buildings with a large number of banners placed around, such as a city, castle or something along the lines. Results may depend on hardware and the number of banners + number of patterns placed.

How to reproduce

1. Create a void world.
2. Fill a certain area with any banner with no patterns and check your FPS graph (ALT+F3), replace with the coords you want.

   /fill x1 y1 z1 x2 y2 z2 minecraft:blue_banner
   

3. Now fill the same area with this banner (16 layers), use the same coords as before:

   /fill x1 y1 z1 x2 y2 z2 minecraft:blue_banner{Patterns:[{Pattern:ss,Color:15},{Pattern:cbo,Color:11},{Pattern:tts,Color:15},{Pattern:bts,Color:15},{Pattern:mr,Color:11},{Pattern:bo,Color:15},{Pattern:flo,Color:15},{Pattern:mc,Color:3},{Pattern:ss,Color:15},{Pattern:cbo,Color:11},{Pattern:tts,Color:15},{Pattern:bts,Color:15},{Pattern:mr,Color:11},{Pattern:bo,Color:15},{Pattern:flo,Color:15},{Pattern:mc,Color:3}]}
   

4. Compare the frame rate.