Like the sibling comment says, while there are alternative flash players, they are either unmaintained (Shumway since 2016) or not feature complete yet (Ruffle)

Like the sibling comment says, while there are alternative flash players, they are either unmaintained (Shumway since 2016) or not feature complete yet (Ruffle). Since performance was stated as the main goal, a bespoke solution that only supports what the games actually use is probably the best performing.