Why do top conferences/journals between different research fields sometimes have a large difference in h5-index?

Question

Example: the top conferences/journals for computer vision have an h5-index almost twice higher than the top conferences/journals for natural language processing:

https://scholar.google.com/citations?view_op=top_venues&hl=en&vq=eng_computervisionpatternrecognition (mirror 1, mirror 2):

Index	Publication	h5-index	h5-median
1.	IEEE/CVF Conference on Computer Vision and Pattern Recognition	299	509
2.	IEEE/CVF International Conference on Computer Vision	176	295
3.	European Conference on Computer Vision	144	286
4.	IEEE Transactions on Pattern Analysis and Machine Intelligence	131	261
5.	IEEE Transactions on Image Processing	113	156
6.	Pattern Recognition	85	126
7.	IEEE Computer Society Conference on Computer Vision and Pattern Recognition Workshops	73	110
8.	International Journal of Computer Vision	70	150
9.	Medical Image Analysis	67	115
10.	Pattern Recognition Letters	59	80

https://scholar.google.com/citations?view_op=top_venues&hl=en&vq=eng_computationallinguistics (mirror 1, mirror 2):

#	Publication	h5-index	h5-median
1.	Meeting of the Association for Computational Linguistics (ACL)	135	220
2.	Conference on Empirical Methods in Natural Language Processing (EMNLP)	112	197
3.	Conference of the North American Chapter of the Association for Computational Linguistics: Human Language Technologies (HLT-NAACL)	90	148
4.	Transactions of the Association for Computational Linguistics	53	112
5.	International Workshop on Semantic Evaluation	50	93
6.	International Conference on Computational Linguistics (COLING)	49	73
7.	Conference of the European Chapter of the Association for Computational Linguistics (EACL)	45	68
8.	Conference on Computational Natural Language Learning (CoNLL)	39	68
9.	International Conference on Language Resources and Evaluation (LREC)	38	54
10.	Workshop on Machine Translation	35	64

Answer 1

Different fields have different sizes, speed and publication culture. Thus, it is completely expected that h5-indices will vary a lot between fields. This effect is going to be more pronounced when larger values are considered.

Size: To push an h5-index from eg 199 to 200, the additional paper needs to be cited 200 times within 5 years. In a smaller field, this could easily mean getting citations from an absurdly high fraction of the new papers published.

Speed: The h5-index only counts citations in a 5-year window. In a fast moving field, that doesn't matter much. In a slower-moving field, this can easily ignore most of the potential citations a paper could get.

Publication culture: How big or small are the units people publish? How many references does a typical paper have? Do authors always explicitly cite the masters, or only if really needed?