Iteration over subsets of OrderedSet

[termi-official]

In my application I split up ordered sets into N chunks of approximately same size with ranges. Each of these chunks is then passed into one a single worker (e.g. thread) as an index to some task item. Now I have the issue that getindex has been deprecated and I can not figure how to reproduce this logic without the function (under the constraint that the access for each item should be at most $O(log(N))$ and non-allocating). Could someone give me a hint how the functionality above is supposed to be reproduced without using getindex?