cli: Optimize for large multi-line string outputs

command/format/diff.go

@@ -901,6 +901,17 @@ func (p *blockBodyDiffPrinter) writeValueDiff(old, new cty.Value, indent int, pa
 				}
 			}
 
+			// Optimization for strings which are exactly equal: just print
+			// directly without calculating the sequence diff. This makes a
+			// significant difference when this code path is reached via a
+			// writeValue call with a large multi-line string.
+			if oldS == newS {
+				for _, line := range newLines {
+					p.buf.WriteString(strings.Repeat(" ", indent+4))
+					p.buf.WriteString(line.AsString())
+					p.buf.WriteString("\n")
+				}
+			} else {
 				diffLines := ctySequenceDiff(oldLines, newLines)
 				for _, diffLine := range diffLines {
 					p.buf.WriteString(strings.Repeat(" ", indent+2))
@@ -919,6 +930,7 @@ func (p *blockBodyDiffPrinter) writeValueDiff(old, new cty.Value, indent int, pa
 					}
 					p.buf.WriteString("\n")
 				}
+			}
 
 			p.buf.WriteString(strings.Repeat(" ", indent)) // +4 here because there's no symbol
 			p.buf.WriteString("EOT")