diff --git a/.gitignore b/.gitignore
index 3905ee6b..6870fe28 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,5 +22,6 @@ tests/.cache/*
 *.pot
 /*venv*
 *.mypy_cache
+.eggs
 /plumbum/version.py
 /tests/nohup.out
diff --git a/plumbum/commands/processes.py b/plumbum/commands/processes.py
index c0901017..da33b76e 100644
--- a/plumbum/commands/processes.py
+++ b/plumbum/commands/processes.py
@@ -113,13 +113,14 @@ class ProcessExecutionError(OSError):
     well as the command line used to create the process (``argv``)
     """
 
-    def __init__(self, argv, retcode, stdout, stderr, message=None):
+    def __init__(self, argv, retcode, stdout, stderr, message=None, *, host=None):
 
         # we can't use 'super' here since OSError only keeps the first 2 args,
         # which leads to failuring in loading this object from a pickle.dumps.
         Exception.__init__(self, argv, retcode, stdout, stderr)
 
         self.message = message
+        self.host = host
         self.argv = argv
         self.retcode = retcode
         if isinstance(stdout, bytes):
@@ -143,6 +144,8 @@ def __str__(self):
             lines = ["Unexpected exit code: ", str(self.retcode)]
         cmd = "\n              | ".join(cmd.splitlines())
         lines += ["\nCommand line: | ", cmd]
+        if self.host:
+            lines += ["\nHost:         | ", self.host]
         if stdout:
             lines += ["\nStdout:       | ", stdout]
         if stderr:
diff --git a/plumbum/machines/base.py b/plumbum/machines/base.py
index a498c7c2..754852be 100644
--- a/plumbum/machines/base.py
+++ b/plumbum/machines/base.py
@@ -91,3 +91,12 @@ def __getattr__(self, name):
     @property
     def cmd(self):
         return self.Cmd(self)
+
+    def clear_program_cache(self):
+        """
+        Clear the program cache, which is populated via ``machine.which(progname)`` calls.
+
+        This cache speeds up the lookup of a program in the machines PATH, and is particularly
+        effective for RemoteMachines.
+        """
+        self._program_cache.clear()
diff --git a/plumbum/machines/local.py b/plumbum/machines/local.py
index 723c72c5..a9a4e5f4 100644
--- a/plumbum/machines/local.py
+++ b/plumbum/machines/local.py
@@ -8,6 +8,7 @@
 from contextlib import contextmanager
 from subprocess import PIPE, Popen
 from tempfile import mkdtemp
+from typing import Dict, Tuple
 
 from plumbum.commands import CommandNotFound, ConcreteCommand
 from plumbum.commands.daemons import posix_daemonize, win32_daemonize
@@ -141,6 +142,7 @@ class LocalMachine(BaseMachine):
 
     custom_encoding = sys.getfilesystemencoding()
     uname = platform.uname()[0]
+    _program_cache: Dict[Tuple[str, str], LocalPath] = {}
 
     def __init__(self):
         self._as_user_stack = []
@@ -182,6 +184,14 @@ def which(cls, progname):
 
         :returns: A :class:`LocalPath <plumbum.machines.local.LocalPath>`
         """
+
+        key = (progname, cls.env.get("PATH", ""))
+
+        try:
+            return cls._program_cache[key]
+        except KeyError:
+            pass
+
         alternatives = [progname]
         if "_" in progname:
             alternatives.append(progname.replace("_", "-"))
@@ -189,6 +199,7 @@ def which(cls, progname):
         for pn in alternatives:
             path = cls._which(pn)
             if path:
+                cls._program_cache[key] = path
                 return path
         raise CommandNotFound(progname, list(cls.env.path))
 
diff --git a/plumbum/machines/remote.py b/plumbum/machines/remote.py
index df86fa42..097652d5 100644
--- a/plumbum/machines/remote.py
+++ b/plumbum/machines/remote.py
@@ -173,6 +173,7 @@ def __init__(self, encoding="utf8", connect_timeout=10, new_session=False):
         self.uname = self._get_uname()
         self.env = RemoteEnv(self)
         self._python = None
+        self._program_cache = {}
 
     def _get_uname(self):
         rc, out, _ = self._session.run("uname", retcode=None)
@@ -225,6 +226,13 @@ def which(self, progname):
 
         :returns: A :class:`RemotePath <plumbum.path.local.RemotePath>`
         """
+        key = (progname, self.env.get("PATH", ""))
+
+        try:
+            return self._program_cache[key]
+        except KeyError:
+            pass
+
         alternatives = [progname]
         if "_" in progname:
             alternatives.append(progname.replace("_", "-"))
@@ -233,6 +241,7 @@ def which(self, progname):
             for p in self.env.path:
                 fn = p / name
                 if fn.access("x") and not fn.is_dir():
+                    self._program_cache[key] = fn
                     return fn
 
         raise CommandNotFound(progname, self.env.path)
diff --git a/plumbum/machines/session.py b/plumbum/machines/session.py
index 21d13217..f292eded 100644
--- a/plumbum/machines/session.py
+++ b/plumbum/machines/session.py
@@ -72,7 +72,8 @@ class SessionPopen(PopenAddons):
     """A shell-session-based ``Popen``-like object (has the following attributes: ``stdin``,
     ``stdout``, ``stderr``, ``returncode``)"""
 
-    def __init__(self, proc, argv, isatty, stdin, stdout, stderr, encoding):
+    def __init__(self, proc, argv, isatty, stdin, stdout, stderr, encoding, *, host):
+        self.host = host
         self.proc = proc
         self.argv = argv
         self.isatty = isatty
@@ -132,6 +133,7 @@ def communicate(self, input=None):  # pylint: disable=redefined-builtin
                         stdout,
                         stderr,
                         message="Incorrect username or password provided",
+                        host=self.host,
                     ) from None
                 if returncode == 6:
                     raise HostPublicKeyUnknown(
@@ -140,6 +142,7 @@ def communicate(self, input=None):  # pylint: disable=redefined-builtin
                         stdout,
                         stderr,
                         message="The authenticity of the host can't be established",
+                        host=self.host,
                     ) from None
                 if returncode != 0:
                     raise SSHCommsError(
@@ -148,6 +151,7 @@ def communicate(self, input=None):  # pylint: disable=redefined-builtin
                         stdout,
                         stderr,
                         message="SSH communication failed",
+                        host=self.host,
                     ) from None
                 if name == "2":
                     raise SSHCommsChannel2Error(
@@ -156,6 +160,7 @@ def communicate(self, input=None):  # pylint: disable=redefined-builtin
                         stdout,
                         stderr,
                         message="No stderr result detected. Does the remote have Bash as the default shell?",
+                        host=self.host,
                     ) from None
 
                 raise SSHCommsError(
@@ -164,6 +169,7 @@ def communicate(self, input=None):  # pylint: disable=redefined-builtin
                     stdout,
                     stderr,
                     message="No communication channel detected. Does the remote exist?",
+                    host=self.host,
                 ) from err
             if not line:
                 del sources[i]
@@ -202,7 +208,10 @@ class ShellSession:
                             is seen, the shell process is killed
     """
 
-    def __init__(self, proc, encoding="auto", isatty=False, connect_timeout=5):
+    def __init__(
+        self, proc, encoding="auto", isatty=False, connect_timeout=5, *, host=None
+    ):
+        self.host = host
         self.proc = proc
         self.custom_encoding = proc.custom_encoding if encoding == "auto" else encoding
         self.isatty = isatty
@@ -299,6 +308,7 @@ def popen(self, cmd):
             MarkedPipe(self.proc.stdout, marker),
             MarkedPipe(self.proc.stderr, marker),
             self.custom_encoding,
+            host=self.host,
         )
         return self._current
 
diff --git a/plumbum/machines/ssh_machine.py b/plumbum/machines/ssh_machine.py
index 8ce41c43..648b0efb 100644
--- a/plumbum/machines/ssh_machine.py
+++ b/plumbum/machines/ssh_machine.py
@@ -104,6 +104,7 @@ def __init__(
 
         scp_args = []
         ssh_args = []
+        self.host = host
         if user:
             self._fqhost = f"{user}@{host}"
         else:
@@ -208,6 +209,7 @@ def session(self, isatty=False, new_session=False):
             self.custom_encoding,
             isatty,
             self.connect_timeout,
+            host=self.host,
         )
 
     def tunnel(