1.) Ordering Items: The Item Number field is a text field because it often has both numbers and letters (for example, "432Furn"). Because of this numbers are ordered differently. So '10' or '11' will come before 2,3,4,5,6,7,8,9.
The best way to number Items then is use the same number of characters for all numbers. For example, if you have a project with more than 100 Items to use 3 characters for all numbers: 001,002,055,099,125,678, etc. This will order the list in a more predictable fashion.
2.) Speed Items & Work list loading time: To speed up Items page loading
it's best to filter the list by Area, SalesCode, Proposal, etc. This way you're only loading the part of the project you're working on and it's much faster.
Use the 'Show Per Page' setting at the bottom left of the list to adjust the number if you need more Items (50,100, 250) per page.
Right-click on the Add New Item link to open it in a new tab and only refresh the main Items List when you're ready.