most common interface to Tcl, the call() method, by maybe 20-25%.

The speedup code avoids the construction of a Tcl command string from
the argument list -- the Tcl argument list is immediately parsed back
by Tcl_Eval() into a list that is *guaranteed* (by Tcl_Merge()) to be
exactly the same list, so instead we look up the command info and call
the command function directly.  If the lookup fails, we fall back to
the old method (Tcl_Merge() + Tcl_Eval()) so we don't need to worry
about special cases like undefined commands or the occasional command
("after") that sets the info.proc pointer to NULL -- let TclEval()
deal with these.