strace'ing journald while calling "logger foo" reveals:

clock_gettime(CLOCK_BOOTTIME, {124771, 959412867}) = 0
ioctl(3, FIONREAD, [30])                = 0
recvmsg(3, {msg_name(0)={sa_family=AF_UNSPEC, sa_data="\0\0\0\0\0\0\0\0\0\0\0\0\0\0"}, msg_iov(1)=[{"<13>Jan 21 17:55:28 smurf: foo", 24575}], msg_controllen=64, {cmsg_len=32, cmsg_level=SOL_SOCKET, cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=MSG_CMSG_CLOEXEC}, MSG_DONTWAIT|MSG_CMSG_CLOEXEC) = 30
sendmsg(3, {msg_name(29)={sa_family=AF_LOCAL, sun_path="/run/systemd/journal/syslog"}, msg_iov(1)=[{"<13>Jan 21 17:55:28 smurf: foo", 30}], msg_controllen=28, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=14495, uid=501, gid=501}}, msg_flags=0}, MSG_NOSIGNAL) = -1 ESRCH (No such process)
sendmsg(3, {msg_name(29)={sa_family=AF_LOCAL, sun_path="/run/systemd/journal/syslog"}, msg_iov(1)=[{"<13>Jan 21 17:55:28 smurf: foo", 30}], msg_controllen=28, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=579, uid=501, gid=501}}, msg_flags=0}, MSG_NOSIGNAL) = -1 ECONNREFUSED (Connection refused)
open("/proc/14495/cgroup", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/proc/14495/comm", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
readlinkat(AT_FDCWD, "/proc/14495/exe", 0x7fa6e89c8b30, 99) = -1 ENOENT (No such file or directory)
open("/proc/14495/cmdline", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/proc/14495/status", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/proc/14495/sessionid", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/proc/14495/loginuid", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/proc/14495/cgroup", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
ftruncate(18, 16777216)                 = 0
ioctl(3, FIONREAD, [0])                 = 0
recvmsg(3, 0x7fff18b3e670, MSG_DONTWAIT|MSG_CMSG_CLOEXEC) = -1 EAGAIN (Resource temporarily unavailable)
epoll_wait(8,
^C

That first sendmsg() call returns ESRCH. Therefore, all these accesses to /proc/ are by definition superfluous overhead. Please don't do them.