[Maps] Typing in custom color maps is slow

[thomasneirynck]

Typing in values in a custom color map can be slow. This applies to either custom ordinal ramps or custom categorical color palettes.

Steps

1. add a document layer with lines or polygons (do not use points. The bug is not really present for point-only data)
2. create a by-value color-style, and open custom color ramp or palette.
3. start typing in values
   -> note that the characters do not appear at interactive speeds, but instead come in slow.

This issue is present in 7.5, since custom ramps were introduced.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[thomasneirynck]

Closed by #56446