Control-click the flags to (de)select multiple flags.
The --big-transactions flag is important for two
of the bigger tests. Without it, those tests create many
thousands of implicit transactions, reducing the affected
tests to an absolute crawl, in particular with OPFS.
The easiest way to try different optimization levels is,
from this directory:
$ rm -f jswasm/speedtest1.js; make -e emcc_opt='-O2' speedtest1
Then reload this page. -O2 seems to consistently produce the fastest results.