title: GPUImage increasing video filter speed for iphone 4
body: <p>I'm using GPUImage to apply filters to videos, and everything works fine on iPhone 4S and up, but on iPhone 4, things get really slow (it takes 30-40 seconds to apply filter to a 6 second video). Audio is disabled while applying filters</p>
  
  <pre><code>movieFile.audioEncodingTarget = nil;
  _movieWriter.shouldPassthroughAudio= YES;
  </code></pre>
  
  <p>is there a way for me to increase the performance any further on iphone 4? I tried decreasing the output size but it did not do much. Can I remove the audio code completely from GPUImage to increase performance since I'm adding the audio later on to the movie file?</p>
  
  <p>Sample code:</p>
  
  <pre><code>//Initialize the filter
  _filter = [[GPUImageContrastFilter alloc] init];
  [(GPUImageContrastFilter *)_filter setContrast:1.75];
  
  finalMoviePath = [NSString stringWithFormat:@"%@%s", NSTemporaryDirectory(), filteredVideoFileName];
  _finalMovieURL = [NSURL fileURLWithPath:finalMoviePath];
  
  //Delete Previous Filtered Video File
  if ([[NSFileManager defaultManager] fileExistsAtPath:finalMoviePath]) {
      if (![[NSFileManager defaultManager] removeItemAtPath:finalMoviePath error:&amp;error]) {
      }
  }
  
  //Create the GPUImageMovie
  movieFile = [[GPUImageMovie alloc] initWithURL:video.URL];
  movieFile.runBenchmark = NO;
  
  [movieFile addTarget:_filter];
  
  //MovieWriter
  _movieWriter = [[GPUImageMovieWriter alloc] initWithMovieURL:_finalMovieURL size:movieSize];
  movieFile.audioEncodingTarget = nil;
  _movieWriter.shouldPassthroughAudio= YES;
  
  movieFile.playAtActualSpeed = NO;
  [movieFile startProcessing];
  [_filter addTarget:_movieWriter];
  
  [_movieWriter startRecording];
  
  
  [_movieWriter setCompletionBlock:^{  
      //Completion stuff
  }];
  </code></pre>