I've noticed that Poncirus hybrid seedlings are slower growing than other seedlings.
I've been growing a large number of different seedlings, so I think I can make this observation.
I've grown 2 seedlings from US 852 (Changsha mandarin x trifoliate), 3 seedlings from Tai-tri (taiwanica x trifoliate), 2 seedlings from N1tri (ichangensis x trifoliate), and without exception they have all seemed to grow very slow, certainly relative to other seedlings.
I've also been growing numerous seedlings from Ichangquat (kumquat x ichangensis) and a few cuttings from C. ichangensis. The Ichangquat grows very vigorously, even more vigorously than Yuzu, which also grows vigorously. The C. ichangensis cuttings have grown relatively slow, but reliably, with steady, healthy, and continuous growth. I do also have one seedling from Thomasville citrangequat, it has grown about as fast as kumquat, only about medium vigor. Unfortunately I don't have any pure trifoliate seedlings to compare to.
I think this should not be surprising. When two different plant species are hybridized together that are farther apart from each other in terms of relation, the result is often offspring with a slower growth rate and slightly stunted, or often with generally poorer health. It's presumably due to slight incompatibility between the genetic makeup of the two parents, since they are so distantly related (This is termed outbreeding depression )
Obviously Poncirus trifoliata is less related to the general citrus family than other citrus species are.
For comparison, taiwanica is vigorous and fast growing, and Changsha mandarin is a little slower growing but its growth is very reliable and healthy.
C. ichangensis is also slower growing (slower than Changsha mandarin) yet with very healthy reliable steady growth.
The trifoliate hybrid seedlings are all even slower growing than C. ichangensis, and do not have as healthy reliable robust growth.
What's interesting here is that since I am growing seedlings of hybrids, we are possibly getting to examine the effects in the F2 generation.
I believe some of these seedlings may not be nucellar.
I know citrangequat is said to always have nucellar seed, but my seedling (I harvested it from the fruit myself) seems to have mostly normal unifoliate leaves, with only two malformed bifoliate leaves, reminiscent that it has some trifoliate parentage in its ancestry.
Around half the Ichangquat seedlings appear to have obviously variable leaf morphology, so are almost certainly zygotic.
I do also have a Dimicelli cutting (probably either a citrandarin, F2 citrandarin seedling, or maybe second generation citrandarin cross with the tangor 'Temple Orange' , its exact origins are a little ambiguous) and it has been growing rather slowly, though with steady reliable growth.
The only trifoliate hybrid I have grown with vigorous robust growth is Duncan citrumelo (trifoliate x grapefruit) but even it is not as fast growing as grapefruit (in warm growing conditions).
I know this is hardly a controlled scientific study, but I believe with the number of seedlings I have been growing, this is strongly anecdotal, and this generalized observation may have value.
Maybe someone here (I am sure there are plenty) who has grown Poncirus seedlings can comment on how they grow compared to other citrus seedlings.
If Poncirus hybrids tend to be much slower growing, it may be of particular importance to try to select the most vigorous seedlings in hybridization attempts. I believe there is a strong correlation between level of vigor and ability to recover after cold damage.