Without the change:
$ sudo lockstat -e 0,1 sleep 5
Adaptive mutex spin: 42 events in 5.013 seconds (8 events/sec)
Count indv cuml rcnt nsec Lock Caller
-------------------------------------------------------------------------------
12 29% 29% 0.00 14237 0xfffffe0001430990 usbd_transfer_pending+0xa2
5 12% 40% 0.00 518 0xfffff8000de18520 doselwakeup+0x132
4 10% 50% 0.00 14339 0xfffff8000d431090 usbd_do_request_flags+0x5e3
3 7% 57% 0.00 1595 0xfffff8000d431090 usb_callback_proc+0xd9
2 5% 62% 0.00 23216 Giant usbd_enum_lock+0x10f
2 5% 67% 0.00 243 0xfffff800246f14c0 sopoll_generic+0x84
1 2% 69% 0.00 579 Giant _cv_wait+0x226
1 2% 71% 0.00 2151 0xfffffe000228f570 selrecord+0xd8
1 2% 74% 0.00 37062 0xfffffe0001430990 usbd_do_request_flags+0x872
1 2% 76% 0.00 768 0xfffff8043a8cf580 uipc_send+0x8bf
1 2% 79% 0.00 590 0xfffff8043a2046a8 uipc_send+0x927
1 2% 81% 0.00 667 0xfffff80442e07008 vhpet_mmio_read+0x49
1 2% 83% 0.00 2047 0xfffffe0001430990 xhci_interrupt+0x75
1 2% 86% 0.00 986 0xfffffe0001430990 usb_process+0x167
1 2% 88% 0.00 354 0xfffff8000de18520 seltdwait+0x78
1 2% 90% 0.00 485 0xfffff8000d384090 usb_callback_proc+0xd9
1 2% 93% 0.00 335 0xfffff800242b4c78 soreceive_generic+0x1c0
1 2% 95% 0.00 601 0xfffff8006b4c4c08 ttydev_enter+0x64
1 2% 98% 0.00 5623 0xfffff802286ff808 ttydev_enter+0x64
1 2% 100% 0.00 423 0xfffff802286ff808 ptsdev_ioctl+0x75c
-------------------------------------------------------------------------------
With the change:
$ sudo lockstat -e 0,1 sleep 5
Adaptive mutex spin: 1325 events in 5.031 seconds (263 events/sec)
Count indv cuml rcnt nsec Lock Caller
-------------------------------------------------------------------------------
560 42% 42% 0.00 1111 vm page free queue vm_page_free_toq+0xe6
558 42% 84% 0.00 937 vm page free queue vm_page_alloc+0xbb
51 4% 88% 0.00 687 vm active pagequeue vm_page_enqueue+0x81
42 3% 91% 0.00 878 vm active pagequeue vm_page_dequeue+0x7b
26 2% 93% 0.00 1146 vm inactive pagequeue _vm_page_deactivate+0xf7
19 1% 95% 0.00 823 vm inactive pagequeue vm_page_dequeue+0x7b
10 1% 96% 0.00 283 pmap pv chunk list get_pv_entry+0x19c
10 1% 96% 0.00 2405 pmap pmap_extract_and_hold+0xe7
9 1% 97% 0.00 464 sellck doselwakeup+0x132
4 0% 97% 0.00 9202 xhci0 usbd_transfer_pending+0xa2
4 0% 98% 0.00 421 process lock trap_pfault+0x27a
4 0% 98% 0.00 6477 USB device mutex usbd_do_request_flags+0x5e3
3 0% 98% 0.00 383 vm page vm_page_rename+0xdc
3 0% 98% 0.00 305 vm page vm_fault_hold+0x1fad
3 0% 99% 0.00 352 pmap pv chunk list free_pv_chunk+0x66
3 0% 99% 0.00 3283 Giant usbd_enum_lock+0x10f
2 0% 99% 0.00 453 vm page vm_fault_hold+0x5fb
2 0% 99% 0.00 548 so_rcv soreceive_generic+0x981
2 0% 99% 0.00 609 so_rcv soreceive_generic+0x1c0
1 0% 99% 0.00 418 vm page release_page+0xba
1 0% 99% 0.00 216 unp_mtx uipc_send+0x8bf
1 0% 99% 0.00 342 ui_vmsize swap_release_by_cred+0x149
1 0% 100% 0.00 11922 ttymtx ttydev_enter+0x64
1 0% 100% 0.00 314 vhpet lock vhpet_mmio_read+0x49
1 0% 100% 0.00 399 cdev devvn_refthread+0x9e
1 0% 100% 0.00 353 USB device mutex usb_callback_proc+0xd9
1 0% 100% 0.00 1225 process lock trap_pfault+0x1b3
1 0% 100% 0.00 1239 process lock ast+0x42b
1 0% 100% 0.00 418 so_snd sopoll_generic+0x84
-------------------------------------------------------------------------------