diff --git a/fs/operations/operations.go b/fs/operations/operations.go
index d2220cc42..88daa6c91 100644
--- a/fs/operations/operations.go
+++ b/fs/operations/operations.go
@@ -546,7 +546,11 @@ func SuffixName(ctx context.Context, remote string) string {
 // and accumulating stats and errors.
 //
 // If backupDir is set then it moves the file to there instead of
-// deleting
+// deleting.
+//
+// Use BackupDir to find backupDir from --backup-dir. That lookup is
+// relatively expensive, so when deleting many files do it once outside
+// the loop rather than calling it for every object.
 func DeleteFileWithBackupDir(ctx context.Context, dst fs.Object, backupDir fs.Fs) (err error) {
 	tr := accounting.Stats(ctx).NewCheckingTransfer(dst, "deleting")
 	defer func() {
@@ -579,8 +583,8 @@ func DeleteFileWithBackupDir(ctx context.Context, dst fs.Object, backupDir fs.Fs
 
 // DeleteFile deletes a single file respecting --dry-run and accumulating stats and errors.
 //
-// If useBackupDir is set and --backup-dir is in effect then it moves
-// the file to there instead of deleting
+// DeleteFile does not honour --backup-dir: it always deletes. Call
+// DeleteFileWithBackupDir instead if --backup-dir support is required.
 func DeleteFile(ctx context.Context, dst fs.Object) (err error) {
 	return DeleteFileWithBackupDir(ctx, dst, nil)
 }