Without spending too much time on it, I think a cursory check shows git doing "the smart thing" with ignore rules.
$ mkdir foo
$ cd foo
$ mkdir $(seq 1 1000)
$ git init .
$ strace -c git status
On branch master
No commits yet
nothing to commit (create/copy files and use "git add" to track)
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
75.97 0.058830 57 1035 15 openat
16.48 0.012764 6 2002 getdents
3.87 0.002999 3 1021 close
2.91 0.002254 2 1022 fstat
0.19 0.000147 8 18 14 lstat
0.13 0.000100 4 24 read
0.12 0.000092 6 16 12 stat
0.09 0.000072 3 21 14 access
0.06 0.000048 10 5 write
0.04 0.000033 33 1 unlink
0.03 0.000020 3 8 rt_sigaction
0.02 0.000018 2 12 mprotect
0.02 0.000015 15 1 munmap
0.01 0.000010 3 4 getcwd
0.01 0.000006 0 16 mmap
0.01 0.000006 6 1 ioctl
0.01 0.000005 2 3 brk
0.01 0.000004 4 1 chdir
0.00 0.000003 2 2 getpid
0.00 0.000003 3 1 1 readlink
0.00 0.000002 1 2 rt_sigprocmask
0.00 0.000002 2 1 set_tid_address
0.00 0.000002 2 1 set_robust_list
0.00 0.000002 2 1 prlimit64
0.00 0.000000 0 1 execve
0.00 0.000000 0 1 arch_prctl
------ ----------- ----------- --------- --------- ----------------
100.00 0.077437 5221 56 total
$ echo "/*" > .gitignore
$ strace -c git status
On branch master
No commits yet
nothing to commit (create/copy files and use "git add" to track)
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
46.25 0.000568 284 2 getdents
16.78 0.000206 6 35 14 openat
7.41 0.000091 6 16 12 stat
7.17 0.000088 4 21 14 access
5.62 0.000069 3 25 read
4.72 0.000058 3 18 14 lstat
4.15 0.000051 2 22 close
2.93 0.000036 36 1 unlink
2.28 0.000028 1 23 fstat
0.81 0.000010 3 4 getcwd
0.65 0.000008 1 8 rt_sigaction
0.41 0.000005 5 1 ioctl
0.33 0.000004 4 1 chdir
0.24 0.000003 2 2 getpid
0.24 0.000003 3 1 1 readlink
0.00 0.000000 0 5 write
0.00 0.000000 0 16 mmap
0.00 0.000000 0 12 mprotect
0.00 0.000000 0 1 munmap
0.00 0.000000 0 3 brk
0.00 0.000000 0 2 rt_sigprocmask
0.00 0.000000 0 1 execve
0.00 0.000000 0 1 arch_prctl
0.00 0.000000 0 1 set_tid_address
0.00 0.000000 0 1 set_robust_list
0.00 0.000000 0 1 prlimit64
------ ----------- ----------- --------- --------- ----------------
100.00 0.001228 224 55 total