Alright there's quite a massive problem with how splitters affect belt speeds in the current EX build.
To test, I waited until a mk2 storage container filled up with 500 items to prevent any initial transfer-rate inconsistencies from affecting the results.
Tthen, using Premiere pro editing software, skipped ahead exactly 1 minute on the frame to see how many items are in the container at that moment.
I kept the UI of the storage container open the whole time and set the frame to start with 500 items where it flipped from the previous number to 500 for consistency.
I used mk5 conveyorbelts in every test.
Here are the results of my testing.
Test 1: mk1 storage container - splitter - splitter - container.
I only used 1 output of the splitter each time and connected that to the next splitter.
01:05:44 in timeline: 500 items
02:05:44 in timeline: 11 stacks + 15 items = 1115 items
1115 - 500 from start = 615 items per minute.
Test 2: mk1 container - splitter where 1 output has a tiny piece of belt so it gets blocked when items fill it, and the second output connected to a second splitter with identical setup, which then connects to the mk2 storage container
04:14:17 in timeline: 500 items
05:14:17 in timeline: 10 stacks + 87 items = 1087 items
1087 - 500 from start = 587 items per minute.
Test 3: mk1 container - splitter where 2 outputs have a tiny piece of belt so it gets blocked when items fill it, and the third output connected to a merger which then connects to a second splitter with identical setup, connected to another merger which then connects to the mk2 storage container
25:33:41 in timeline: 500 items
26:33:41 in timeline: 10 stacks + 63 items = 1063 items
1063 - 500 from start = 563 items per minute.
Regular mk5 conveyor belt: 780 IPM (Items Per Minute)
Setup 1: 615 IPM = 78.84% of normal IPM
Setup 2: 587 IPM = 75.25% of normal IPM
Setup 3: 563 IPM = 72.18% of normal IPM
Workaround: don't use a splitter if you have 1 output tranferring items.
always add a second output, and having 2 outputs flowing back into a merger after the splitter, to keep the belt-speed fully utilized.
This really needs to be fixed.