Thanks for the details, Matt. As a work around (which you probably already know), you can disable the stats collection by setting that config parameter to 1 or setting phoenix.stats.guidepost.width to a very large value.
Would you mind opening a JIRA and we can continue the discussion there? In particular, I'd be curious about your schema (in particular your primary key constraint, how big your row keys are, and whether or not your table is salted) and the queries over which you're seeing performance degradation. Do your queries have limit clauses on them? Also, are you querying from a JVM that stays up (i.e. like sqlline or an app server) or is the JVM being spun up each time? When the query times degrade, do you get a sense of where the bottleneck is (client vs server CPU)?
Another useful piece of information would be the total number of guideposts which you can get through a query like this:
SELECT SUM(GUIDE_POSTS_ROW_COUNT) FROM SYSTEM.STATS WHERE PHYSICAL_NAME='your_table_name';