I wouldn't call it major, but in certain situations it is noticeable.
I tested the same exact world-gen/seed/everything except on 32 and 64 bit.
I ran them both for three minutes and tracked info about their progress.
32 bit started: 11:21:30, hit stop 11:24:30, finished 11:24:45 on year 184.
32 bit pops:
>56349 Dwarves
>30427 Humans
>48514 Elves
>11572 Goblins
>3913 Kobolds
>Total: 150775
64 bit started: 11:27:30, hit stop 11:30:30, finished 11:30:36 on year 186.
64 bit pops:
>59593 Dwarves
>33106 Humans
>42689 Elves
>12330 Goblins
>10884 Kobolds
>Total: 158602
Then I did two runs for 300 years and checked how long it took them.
test64 start: 10:38:30, stop: 10:54:01, finished: 10:54:34 (year 300), 16:04 total worldgen
64 bit pops:
>60553 Dwarves
>32485 Humans
>51199 Elves
>36119 Goblins
>5738 Kobolds
>Total: 186094
test32 start: 10:56:30, target: 11:12:01 (year 286),stop: 11:13:24, finish: 11:14:05 (year 300), 18:05 total worldgen
32 bit pops:
>54795 Dwarves
>28097 Humans
>45384 Elves
>27181 Goblins
>8280 Kobolds
>Total: 163737
So in summary, 64 bit world-gen reaches the same year earlier, despite having significantly higher populations (23k+ in the 300 year test), responds faster to the stop signal, and takes longer to slow down as badly as the 32 bit world-gen.
This isn't directly applicable to fort mode, but world-gen is one of the most extreme examples of lag you usually encounter outside of like century old megaproject forts or 10k+ goblins and 100k trolls/beak dogs in a dark fort while you collapse it in adventurer mode, and it was easier to quantify the difference due to the bitness change.