I used this test:
console.time();for(var t=0; t<200; t++)
{$tw.wiki.filterTiddlers("[all[tiddlers+shadows]sameday[20170210]]");};c
onsole.timeEnd()
Before this patch, I got speeds of approx 190ms, versus 140ms
afterwards.
Note that the ability to add a cache property like this is only
possible because tiddler objects are immutable.
9fc2086b71