faster method `mul` for graphs #17741

Job	Run time
test-new	14m 27s
test-mod (sagemath_categories-check)	3m 51s
test-long (src/sage/[a-f]*)	33m 52s
test-long (src/sage/[g-o]*)	34m 16s
test-long (src/sage/[p-z]*)	47m 9s
test-long (src/doc src/sage_docbuild src/sage_setup)	9m 51s
coverage-report	17m 8s
	2h 40m 34s

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