Thread::current is supposed to be quick, but we still call it TONS of times. E.g., when running with "java -version", on my machine it executes about 116,000,000 instructions (according to linux "perf stat -r 100 java -version". Thread::current() is called about 130000 times (see patch below), which leaves about less than 1000 instructions between each call to Thread::current(). So even if each call of Thread::current() takes just 10 instructions, that would add up to 1% of total instructions.

Many of these calls are unnecessary. The calls below are supposed to be used for debug builds only, but they are unintentionally included into the product build.

http://hg.openjdk.java.net/jdk/jdk/file/66211c44e126/src/hotspot/share/runtime/mutex.cpp#l159
void Mutex::unlock() {
  assert_owner(Thread::current());
  set_owner(NULL);
  _lock.unlock();
}

void Monitor::notify() {
  assert_owner(Thread::current());
  _lock.notify();
}

void Monitor::notify_all() {
  assert_owner(Thread::current());
  _lock.notify_all();
}

Removing these from the product build reduces the calls by about 15,000 times. On my machine 'java -version' is about 0.6% faster.

Before: 0.0420053188 sec
After: 0.0417649239 sec
~ 99.42770374%

-----------
The patch for counting Thread:current() calls is in the attachment. Actual numbers variy depending on the number of compiler threads, etc.

Before:
$ bin/java -version
java version "14-internal" 2020-03-17
Java(TM) SE Runtime Environment (build 14-internal+0-adhoc.iklam.open)
Java HotSpot(TM) 64-Bit Server VM (build 14-internal+0-adhoc.iklam.open, mixed mode, sharing)
Thread::current() calls = 133886

After:
$ bin/java -version
java version "14-internal" 2020-03-17
Java(TM) SE Runtime Environment (build 14-internal+0-adhoc.iklam.open)
Java HotSpot(TM) 64-Bit Server VM (build 14-internal+0-adhoc.iklam.open, mixed mode, sharing)
Thread::current() calls = 118860