Well, well, well. It seems everyone's being building item sorters incorrectly for six years, then wondering why they cause so much lag.

I've fixed the lag. Well, not all of the lag, but the main cause of it, anyway. It's quite simple. Use a 2-tick delay on the repeater.

When an item sorter uses a 1-tick delay on the repeater, the redstone dust pulses with every item, this causes lighting updates, and this causes the lag. A lot of it. Setting the delay on the repeater to two ticks causes the redstone to have a steady output. No changing levels, no lighting updates, no lag. It will still have lighting updates when different items go through, but when it's the same item you will notice a huge difference.

Try it for yourself. Go to any item sorter you have that has a lot of lag and identify a repeater you can change easily for testing purposes. Get a couple of stacks of the corresponding item and put them through the sorter. LAG. Set that repeater to two ticks. Put a couple of stacks of the item through again. You should notice an FPS improvement.

When I tried this, my item sorter had 12 FPS when running and 20 FPS when not running (it's such a beast that it impacts FPS even when it isn't running). After I made this change, I'm getting 20 FPS. Mostly. It still drops to 12 FPS or lower, but it's occasional. I'm guessing that's when it's switching to a different item. Still, FPS drops occasionally while running is much better than FPS drops all the time.

I also posted a Reddit thread: A simple way to reduce lag with item sorters.

Why it works: adjusting the repeater to 2 ticks synchronises the speed of the redstone to the speed that hoppers transfer items and this reduces redstone updates. Hoppers transfer items once every 0.4 seconds. Comparator = 0.1 seconds, repeater = 0.2 seconds, redstone torch = 0.1 seconds. When redstone is balanced like this, the FPS is better.