If you do some basic level of profiling, I think you can narrow down the problem. For me, I had a similar issue where replaceCurrentItemWithPlayerItem was blocking the UI thread. I resolved it by examining my code to find out which line was taking time. For me the AVAsset loading was taking time. So I used the loadValuesAsynchronouslyForKeys method of AVAsset to resolve my issue.
Hence can you try the following:
-(void)initializeNewObject:(CustomObject*)o
{
//If this cell is being dequeued/re-used, its player might still be playing the old file
[self.player pause];
self.currentObject = o;
/*
//Setting text-variables for title etc. Removed from this post, but by commenting them out in the code, nothing improves.
*/
//Replace the old playerItem in the cell's player
NSURL *url = [NSURL URLWithString:self.currentObject.url];
AVAsset *newAsset = [AVAsset assetWithURL:url];
[newAsset loadValuesAsynchronouslyForKeys:@[@"duration"] completionHandler:^{
AVPlayerItem *newItem = [AVPlayerItem playerItemWithAsset:newAsset];
[self.player replaceCurrentItemWithPlayerItem:newItem];
}];
//The last line above, replaceCurrentItemWithPlayerItem:, is the 'bad guy'.
//When commenting that one out, all lag is gone (of course, no videos will be playing either)
//The lag still occurs even if I never call [self.player play], which leads me
//to believe that nothing after this function can cause the lag
self.isPlaying = NO;
}