The new AbuseFilterCachingParser supports a saner syntax, and it has improved performance . However, it also stopped supporting various features that the old parser used to support; notably:
Whenever a filter tries to use one of the deprecated features, we already emit a deprecation warning (in 1.34). However, in 1.35 we should start emitting notices whenever we find the old parser to be in use (note: this is the default, as specified in extension.json). Then, in 1.36 we should make the new parser be the default, and in 1.37 we should drop the old parser code altogether.
This is because, right now, we have to maintain two different parsers. Some features cannot be reasonably implemented in both parsers unless we introduce more and more hacks. Hence, we should get rid of the old parser as soon as possible. Proposing 1.37 per above, because fixing incompatible filters may take up a decent amount of time and work for wiki users.
 - At least for WMF wikis, the increase has been fairly small. T234427 tracks possible improvements in this area.