Ultimate Underground Belt connector works only with 40-50% because they stops often where they does no items transported.

Increase the buffer size in Mod Settings.

Thanks, that works. Have it now set from 2 to 100. Do you now what I need for 272 items a second? Normally I have one items on a belt and somtimes two. It´s right that I need more Ups when the number is higher?

There are two settings, one is the buffer size, and the other is how often the buffer gets transferred.

Decreasing the update interval (which increases the rate of updates) improves throughput but costs UPS. Increasing buffer size increases throughput but doesn’t cost UPS. It does mean you have more items buffered though, which may not be what you want depending on what the items are.

You have to decide what trade off works best for your other mods, and size of your base and the speed of your computer.