I don't know anything about mapForce, so this will be a rather generic answer...
If you are invoking this from the command line, many operating systems provide a tool which will check system time before and after executing a command line and report the difference with some reasonable degree of accuracy. In Linux or Unix, it's called "time"; I don't know what's recommended elsewhere.
If you can't find or download that tool, or if you're invoking this through an API, you can write your own version of that -- check the system's millisecond or nanosecond clock (which is a standard library call), invoke the thing you want to test (if necessary, run a shell to invoke it from the command line, otherwise invoke it directly), get the system clock again, subtract the two and report time taken.
If your XSLT system supports it, you may want to time stylesheet compilation and stylesheet execution separately, since compilation is a one-time cost if you expect to reuse that stylesheet.