Different UITableView scrolling performance on iPad 2 & iPhone 4s - ios

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Solved
There was a shadow on the self.view layer. After removed it, the tableview scrolls smoothly (56-59fps).
The problem I got is so weird.
I am building a UITableView based Twitter like App. After optimized the table view loading and scrolling, I tested the App (same code & same UI) on both iPad 2 and iPhone 4S. On the iPad 2, I got 56-59fps smooth scrolling animation. But on the iPhone 4S, I got only 43-49fps scrolling animation, which is awful.
This seems not caused by the Retina display. After I remove all images from cells, the problem is still there. The following is the code of cellForRowAtIndexPath
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
// fetch note from the fetched results controller
Note *note = [self.fetchedResultsController objectAtIndexPath:indexPath];
if ([note.photos count] > 0) {
static NSString *CellIdentifier = #"Photo Cell";
PhotoCell *cell = (PhotoCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil){
cell = [[PhotoCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
} else {
[cell clearContents];
}
NSNumber *currentRowNumber = [NSNumber numberWithInt:indexPath.row];
// check if the photos of this cell has been pre loaded
NSDictionary *coverImages = [self.coverImagesForNotes objectForKey:currentRowNumber];
if (coverImages == nil) {
if (self.tableView.dragging == NO && self.tableView.decelerating == NO) {
coverImages = [self loadCoverImagesForCell:indexPath photos:note.photos];
[self.coverImagesForNotes setObject:coverImages forKey:currentRowNumber];
}
}
// assign contents to the cell
[self compositeContentView:cell forNote:note rowNumber:indexPath.row];
// refine
unsigned textContentHeight = [[self.cellTextHeightDict objectForKey:currentRowNumber] integerValue];
unsigned currentPageNumber = [self currentPageNumberAtRow:indexPath.row];
[cell initPhotoVideoView:textContentHeight photos:note.photos coverImages:coverImages showPage:currentPageNumber rowNumber:indexPath.row];
cell.delegate = self;
return cell;
} else {
static NSString *CellIdentifier = #"Simple Cell";
BaseCell *cell = (BaseCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil){
cell = [[OneViewBaseCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}
// assign contents to the cell
[self compositeContentView:cell forNote:note rowNumber:indexPath.row];
return cell;
}
}
Thanks for answers.

Answer 1

Are you using images in your table view cells? This will be more expensive on a retina display. Also if you are using images, try to load them in on a background thread which will reduce the latency.
If you're not using images then you should really look at how you are creating your cells, please post tableView:cellForRowAtIndexPath: method implementation