I don't know of any differences / conscious changes in SOLR 6 / ASS architecture compared to the old SOLR 4 that would have changed this behaviour. In the default configuration it should actually track / generate queries even more frequently than SOLR 4 (CRON set to every 10 seconds instead of every 15 seconds). There may be some coincidental changes regarding caching of some secondary data that might alleviate the symptons a bit, but the core queries should be unchanged... I'll have to take a closer look when I get time.
I checked my Solr6 installation again and you are right, you only underestimated the performance impact:
Solr6 still insanely queries for all the non-existing transactions (i. e. from the last known transaction time until now), but due a "caching" mechanism (which seems like not a real caching, but rather simply "don't query already queried time", with last time kept in memory) it does this only when it gets started.
After this startup, it queries just for the few latest (although logically still non-existent) transactions - and this small difference from Alfresco Solr4 has a tremendous positive performance impact on the Alfresco system, depending on how often you upload something to your Alfresco, of course.
UPDATE: So I have created ALF-22094 today. Hopefully some competent author will look at it and fix it, or at least (s)he will give some technical explanation, although I can't imagine how this issue could be really advocated.