No library used, just a simplest implementation of hash map. Use it as you need.
I'm hearing about this the first time ever, and it seems to me very strange. Also, a single hash map can be organized on top of TDataLine user objects without any additional objects (via additng Next and Hash fields directly to TDataLine). Two maps, however will need another one pair of such properties, which is not so graceful, but also possible, if we speak about the extreme case.
I've only tested with 100k items. I can see the effect. However, it's not clear for me why this happens.