Partitioning a large table has not improved performance, why?
You generally followed a delusion - that partitioning will give you a significant performance boost for queries compared to a standard index.

This is not the case. There is little difference between filtering with an index and with a partition.

Partitions are not there to make your queries faster, but to allow fast DELETE - by swapping out a partition with an empty version of the table. This allows "truncate" style performance for deleting parts of a table - and that is significant. Brutally significant, if you ever experience the long time it may take to delete dozens of gigabytes of data.

There are also insert scenarios where a partition can help.

But for queries - no, the partition will not be better than proper indices. In fact, it will be slower - as there the work is more complex (management of which partitions to access).