docs: document more differences among different backends #10189

Job	Run time
Test Python (1, 3.9)	29m 25s
Test Python (1, 3.12)	41m 11s
Test Python (2, 3.9)	28m 21s
Test Python (2, 3.12)	36m 12s
Test Python (3, 3.9)	28m 5s
Test Python (3, 3.12)	37m 34s
Test Python (4, 3.9)	28m 56s
Test Python (4, 3.12)	38m 18s
Test Python (5, 3.9)	28m 40s
Test Python (5, 3.12)	37m 46s
Test Python (6, 3.9)	28m 40s
Test Python (6, 3.12)	37m 34s
Combine and update integration test durations (3.9)	9s
Combine and update integration test durations (3.12)	5s
Pass testing Python	1s
	6h 40m 57s

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