The speedup you might get for not supporting threads very much depends on your compiler, your OS, your OS thread support (Linux, Windos, VMS, z/OS, HP-UX, AIX, …) and the configuration of your perl. There is no single number that can be pinned to how much slower/faster perl will be (runtime) when built with/without a configuration option.
For the systems I run a smoke-test on, the gain for non-threaded perl is about
See https://tux.nl/perl5/smoke/index.html for smoke results, and a summary at the page bottom regarding performance differences:
threaded is 6.1% slower than non-threaded
DEBUGGING is 2.5% slower than non-DEBUGGING
gcc/g++ is 29.7% slower than cc (cc vs. gcc on non-Li
+nux and gcc vs. g++ on Linux combined)
stdio is 8.5% slower than perlio
Enjoy, Have FUN! H.Merijn