UITableView too slow on main thread - multithreading

Question

I was wondering if there is any way to accelerate/speed up the UItableView/[reload data].
Even though I reload the data on the main thread using:
> [self.tableview performSelectorOnMainThread:#selector(reloadData) withObject:nil waitUntilDone:No]
I still think the results is quite slow (2 to 3 seconds to refresh each time the table is being reloaded, making a bad user experience if the user keep refreshing the table).
For a bit of background here is how it works at the moment.
1) I create my UITableView from data coming from the database on apple starting (until here no problem and I am fine if its taking few seconds).
2) I click on one of the item of the table view, which opens a new screen to modify the elements.
3) Once the elements is modified, I change the database of my table view so that the elements I changed comes at the top of my table view.
4) When I press the back button on the screen to change my elements, I implements the ViewWillAppear of my UITableView and I reload the data on the main thread.
5) What I observer is a latency of about 2/3 seconds to go from one screen to another. Removing the reload data line from my code, make the transition instantaneous. So the problem is really in the reload data.
Each row of my UITableView has the following elements inside:
A background color coming from a file (this is basically a gradient, the file is about 20Ko, it is a PNG, the file is store on the phone)
1 or 2 UIImageView (depending on the status of what I display) (about 5Ko each, very small PNG icons, the file is store on the phone)
3 UILabel
1 UIButton (inside this PNG, there is a picture, the original size of the picture may vary from 200 to 300ko, the original source is coming from internet. I use SDWebImage to load it/cache it)
I would think I may have too much information on each of my cell, as I can see a big difference in loading time when I only have one row, versus having 10rows.
Would like to have some input from the community to see if anything can be done to improve. The best would be to be able to update the UI in the background, so for example, when I am modifying my elements, I do reloaddata in the background, so that when I come back to the previous screen that shows the UITableView my view is already refreshed,
Thanks

Answer 1

Ok I foudn the solution on this website:
http://www.raywenderlich.com/31166/25-ios-app-performance-tips-tricks#drawing
20) Optimize Your Table Views
Table views need to scroll quickly — when they don’t, users really notice the lag.
To keep your table views scrolling smoothly, ensure that you’ve implemented all of the suggestions below:
Reuse cells by setting the correct reuseIdentifier.
Make as many views opaque as possible, including the cell itself.
Avoid gradients, image scaling, and offscreen drawing.
Cache the height of any rows if they aren’t always the same.
If the cell shows content that comes from the web, be sure to make those calls asynchronously and cache the responses. (Personnal note on this one as it was my issue: look at this other website http://keighl.com/post/sdwebimage-uitableviewcell/ that explains how to use SDWebImage in a UITableView. Very useful.
Use shadowPath to set up shadows.
Reduce the number of subviews.
Do as little work as possible in cellForRowAtIndexPath:. If you need to do some work, do it only once and cache the results.
Use the appropriate data structure to hold the information you need. Different structures have different costs for different operations.
Use rowHeight, sectionFooterHeight and sectionHeaderHeight to set constant heights instead of asking the delegate.

Answer 2

You say "A background color coming from a file" - this alerts me that you're doing IO work, which is generally time-consuming, I'm curious if you remmed out the loading a colour from a file if you get a significant speed increase?