UDTs are very good for performance. Arrays of UDTs are especially good. With some constraints. I recommend you read the topics linked above, because they have a great deal of information about how this stuff works. The best summary is probably this one:
Most importantly, for any given base tag in the controller (a controller tag, or a program tag, at the top level), use only one subscription pace. Optimization can only occur for OPC items that are part of the same tag, and at the same pace.
Things that are less important to your operation, and can operate at slower paces, should be in a completely separate tag hierarchy, preferably another array of UDTs.
If you must use AOI tags, and cannot do any InOut parameters nor tweak AOI locals into a nested UDT, you probably want to use my alternate driver.