Emacs 29 master
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Empty ‘directory-abbrev-alist’
Local | Elapsed time: 0.082094s (0.012981s in 1 GCs)
Tramp | Elapsed time: 0.570441s (0.175013s in 13 GCs)

100 items in ‘directory-abbrev-alist’ (no matches)
Local | Elapsed time: 0.334320s (0.129351s in 10 GCs)
Tramp | Elapsed time: 0.838366s (0.294330s in 22 GCs)

100 items in ‘directory-abbrev-alist’ (all matches)
Local | Elapsed time: 0.530187s (0.320165s in 25 GCs)
Tramp | Elapsed time: 0.836190s (0.285371s in 22 GCs)

500 items in ‘directory-abbrev-alist’ (no matches)
Local | Elapsed time: 1.089229s (0.492225s in 38 GCs)
Tramp | Elapsed time: 1.587351s (0.649014s in 50 GCs)

500 items in ‘directory-abbrev-alist’ (all matches)
Local | Elapsed time: 1.292103s (0.687631s in 53 GCs)
Tramp | Elapsed time: 1.592210s (0.651090s in 50 GCs)


With patch
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Empty ‘directory-abbrev-alist’
Local | Elapsed time: 0.076136s (0.012949s in 1 GCs)
Tramp | Elapsed time: 0.510445s (0.160052s in 12 GCs)

100 items in ‘directory-abbrev-alist’ (no matches)
Local | Elapsed time: 0.342509s (0.130916s in 10 GCs)
Tramp | Elapsed time: 0.780201s (0.281118s in 21 GCs)

100 items in ‘directory-abbrev-alist’ (all matches)
Local | Elapsed time: 0.538353s (0.323898s in 25 GCs)
Tramp | Elapsed time: 0.699262s (0.245942s in 19 GCs)

500 items in ‘directory-abbrev-alist’ (no matches)
Local | Elapsed time: 1.104786s (0.500880s in 38 GCs)
Tramp | Elapsed time: 1.524662s (0.642236s in 49 GCs)

500 items in ‘directory-abbrev-alist’ (all matches)
Local | Elapsed time: 1.299160s (0.687867s in 53 GCs)
Tramp | Elapsed time: 1.519829s (0.651691s in 47 GCs)