trying out laurian's performance fix

 #150 (comment)

package.json

@@ -56,6 +56,7 @@
     "prop-types": "^15.6.2",
     "react-keyboard-shortcuts": "^1.1.3",
     "react-simple-tooltip": "^2.3.3",
+    "react-visibility-sensor": "^5.1.1",
     "sbd": "^1.0.15",
     "smpte-timecode": "^1.2.3"
   },

packages/components/timed-text-editor/WrapperBlock.js

@@ -1,5 +1,6 @@
 import React from 'react';
 import { EditorBlock, Modifier, EditorState, SelectionState } from 'draft-js';
+import VisibilitySensor from 'react-visibility-sensor';
 
 import SpeakerLabel from './SpeakerLabel';
 // import { shortTimecode, secondsToTimecode } from '../../Util/timecode-converter/';
@@ -146,7 +147,12 @@ class WrapperBlock extends React.Component {
           {this.props.blockProps.showTimecodes ? timecodeElement : ''}
         </div>
         <div className={ style.text }>
-          <EditorBlock { ...this.props } />
+          <VisibilitySensor partialVisibility={ true }>
+            {({ isVisible }) => isVisible ?
+              <EditorBlock { ...this.props } />
+              : <div contentEditable={ false }>{ this.props.block.text }</div>
+            }
+          </VisibilitySensor>
         </div>
       </div>
     );