There is an optimization made to is_locked that checks if a file is oplocked and if that is the case, it does not need to do more extensive checks. It has been committed as :  https://github.com/samba-team/samba/commit/822fcec39d6597524a94b0e8b9c244d5eb62b972

Unfortunately this optimization does not carry over to leases. So when smb2 leases = yes (and in more or less recent versions of samba it is a default setting) there is much more time spent on locking. 

e.g. 

smb2 leases = yes

Skipping user change - already user
mid 7189, CreditCharge: 1, NeededCharge: 1
smbd_smb2_read: testfile.dat - fnum 3860045402
smb2: read size (4096) too small for minimum aio_read of 0
seqnum=1, fsp->brlock_seqnum=1
is_posix_locked: File testfile.dat, offset = 1933574144, count = 4096, type = READ
posix_lock_in_range: offset_out = 1933574144, count_out = 4096
posix_fcntl_getlock 12 1933574144 4096 0
fcntl_getlock fd=12 op=5 offset=1933574144 count=4096 type=0
fcntl_getlock: fd 12 is returned info 2 pid 0
posix_fcntl_getlock: Lock query call successful
brl_locktest: posix start=1933574144 len=4096 unlocked for fnum 3860045402 file testfile.dat
strict_lock_default: flavour = WINDOWS_LOCK brl start=1933574144 len=4096 unlocked for fnum 3860045402 file testfile.dat
read_file (testfile.dat): pos = 1933574144, size = 4096, returned 4096
smbd_smb2_read: file testfile.dat, fnum 3860045402, offset=1933574144 len=4096 returned 4096
smbd_smb2_read: fnum 3860045402, file testfile.dat, length=4096 offset=1933574144 read=4096
smbd_smb2_request_done_ex: idx[1] status[NT_STATUS_OK] body[16] dyn[yes:4096] at ../source3/smbd/smb2_read.c:164
smb2_set_operation_credit: requested 1, charge 1, granted 1, current possible/max 354/512, total granted/max/low/range 159/8192/7190/159
smbd_smb2_request idx[1] of 5 vectors
smb2_validate_sequence_number: clearing id 7190 (position 7190) from bitmap
smbd_smb2_request_dispatch: opcode[SMB2_OP_READ] mid = 7190



smb2 leases = no

Skipping user change - already user
mid 11722, CreditCharge: 1, NeededCharge: 1
smbd_smb2_read: testfile.dat - fnum 1413864810
smb2: read size (4096) too small for minimum aio_read of 0
is_locked: optimisation - exclusive oplock on file testfile.dat
read_file (testfile.dat): pos = 138412032, size = 4096, returned 4096
smbd_smb2_read: file testfile.dat, fnum 1413864810, offset=138412032 len=4096 returned 4096
smbd_smb2_read: fnum 1413864810, file testfile.dat, length=4096 offset=138412032 read=4096
smbd_smb2_request_done_ex: idx[1] status[NT_STATUS_OK] body[16] dyn[yes:4096] at ../source3/smbd/smb2_read.c:155
smb2_set_operation_credit: requested 1, charge 1, granted 1, current possible/max 354/512, total granted/max/low/range 159/8192/11723/159
smbd_smb2_request idx[1] of 5 vectors
smb2_validate_sequence_number: clearing id 11723 (position 3531) from bitmap
smbd_smb2_request_dispatch: opcode[SMB2_OP_READ] mid = 11723