diff --git a/ui/perfherder/graphs/LegendCard.jsx b/ui/perfherder/graphs/LegendCard.jsx
index 8ef6cdbfc74..b96cc10b0e2 100644
--- a/ui/perfherder/graphs/LegendCard.jsx
+++ b/ui/perfherder/graphs/LegendCard.jsx
@@ -1,9 +1,8 @@
-
+import React from 'react';
 import PropTypes from 'prop-types';
 import { Badge, Button, Form, CloseButton } from 'react-bootstrap';
 
 import { getFrameworkName } from '../perf-helpers/helpers';
-import { graphColors } from '../perf-helpers/constants';
 import { Perfdocs } from '../perf-helpers/perfdocs';
 import GraphIcon from '../../shared/GraphIcon';
 
@@ -22,42 +21,49 @@ const LegendCard = ({
     const newSymbols = [...symbols];
     const errorMessages = [];
     let updates;
-    const newTestData = [...testData].map((item) => {
-      if (item.signature_id === series.signature_id) {
-        const isVisible = !item.visible;
-
-        if (isVisible && newColors.length && newSymbols.length) {
-          item.color = newColors.pop();
-          item.symbol = newSymbols.pop();
-          item.visible = isVisible;
-          item.data = item.data.map((test) => ({
-            ...test,
-            z: item.color[1],
-            _z: item.symbol,
-          }));
-        } else if (!isVisible) {
-          newColors.push(item.color);
-          newSymbols.push(item.symbol);
-          item.color = ['border-secondary', ''];
-          item.symbol = ['circle', 'outline'];
-          item.visible = isVisible;
-          item.data = item.data.map((test) => ({
-            ...test,
-            z: item.color[1],
-            _z: item.symbol,
-          }));
-        } else {
-          errorMessages.push(
-            "The graph supports viewing 6 tests at a time. To select and view a test that isn't currently visible, first deselect a visible test",
-          );
-        }
-      }
-      return item;
-    });
+    const targetIndex = testData.findIndex(
+      (item) => item.signature_id === series.signature_id,
+    );
+    const item = testData[targetIndex];
+    const isVisible = !item.visible;
+    const updatedItem = { ...item };
+
+    if (isVisible && newColors.length && newSymbols.length) {
+      updatedItem.color = newColors.pop();
+      updatedItem.symbol = newSymbols.pop();
+      updatedItem.visible = isVisible;
+      updatedItem.data = item.data.map((test) => ({
+        ...test,
+        z: updatedItem.color[1],
+        _z: updatedItem.symbol,
+      }));
+    } else if (!isVisible) {
+      newColors.push(item.color);
+      newSymbols.push(item.symbol);
+      updatedItem.color = ['border-secondary', ''];
+      updatedItem.symbol = ['circle', 'outline'];
+      updatedItem.visible = isVisible;
+      updatedItem.data = item.data.map((test) => ({
+        ...test,
+        z: updatedItem.color[1],
+        _z: updatedItem.symbol,
+      }));
+    } else {
+      errorMessages.push(
+        "The graph supports viewing 6 tests at a time. To select and view a test that isn't currently visible, first deselect a visible test",
+      );
+    }
 
     if (errorMessages.length) {
       updates = { errorMessages, visibilityChanged: false };
     } else {
+      // rebuild the array by slicing around the updated item
+      const newTestData = [
+        ...testData.slice(0, targetIndex),
+        updatedItem,
+        ...testData.slice(targetIndex + 1),
+      ];
+
       updates = {
         testData: newTestData,
         colors: newColors,
@@ -103,28 +109,39 @@ const LegendCard = ({
 
     newData.splice(index, 1);
 
-    // when removing a test, check to see if the next test in the queue had a color;
-    // if it had secondary and was deselected, reset its color and visibility to
-    // the removed test's color, otherwise push that color back into the colors list
-    if (
-      newData[graphColors.length - 1] &&
-      newData[graphColors.length - 1].color[0] === 'border-secondary'
-    ) {
-      newData[graphColors.length - 1].color = series.color;
-      newData[graphColors.length - 1].visible = true;
-      newData[graphColors.length - 1].data = newData[
-        graphColors.length - 1
-      ].data.map((item) => ({
-        ...item,
-        z: series.color[1],
-      }));
+    // removing a disabled test frees nothing, since it never held a
+    // color, just drop it. Removing a visible test frees its
+    // color: promote the first currently-disabled test to take its
+    // place, or return the color/symbol to the pool if none is waiting.
+    if (series.color[0] === 'border-secondary') {
       resetParams(newData);
-    } else if (series.color[0] === 'border-secondary') {
-      resetParams(newData);
-    } else {
-      const newColors = [...colors, ...[series.color]];
-      resetParams(newData, newColors);
+      return;
     }
+
+    const promoteIndex = newData.findIndex(
+      (item) => item.color[0] === 'border-secondary',
+    );
+
+    if (promoteIndex === -1) {
+      const newColors = [...colors, series.color];
+      const newSymbols = [...symbols, series.symbol];
+      resetParams(newData, newColors, newSymbols);
+      return;
+    }
+
+    const promoted = newData[promoteIndex];
+    newData[promoteIndex] = {
+      ...promoted,
+      color: series.color,
+      symbol: series.symbol,
+      visible: true,
+      data: promoted.data.map((item) => ({
+        ...item,
+        z: series.color[1],
+        _z: series.symbol,
+      })),
+    };
+    resetParams(newData);
   };
 
   const subtitleStyle = 'p-0 mb-0 border-0 text-secondary text-start';
@@ -228,4 +245,11 @@ LegendCard.propTypes = {
   selectedDataPoint: PropTypes.shape({}),
 };
 
-export default LegendCard;
+const areEqual = (prev, next) =>
+  prev.series === next.series &&
+  prev.testData === next.testData &&
+  prev.colors === next.colors &&
+  prev.symbols === next.symbols &&
+  prev.selectedDataPoint === next.selectedDataPoint;
+
+export default React.memo(LegendCard, areEqual);