Good question. I’m going to cite MDL’s analysis to help answer it. As per this page, which was last updated this year, the iQIYI heat index:
“Heat Index measures the investment and popularity of a drama. It takes into account the video views, search, and engagement for a drama. The way it is calculated is not really clear and is proper for the company. So, we can't compare it with other companies' heat index.
Signification of iQiyi heat index (from my interpretation) :
A drama has some popularity when the index reaches 5 500/6 000 (5 500 for dramas that aired all the episodes the same day)
A drama that reaches 9 000 is well known and really popular.
A drama that reached 10k can be considered a super popular and successful drama (top drama from the platform).
10k is not the limit and the score can be higher.
The comparison between dramas of the same company is not significant too. Depending on the actors, the IP's popularity, the drama's budget, etc..., the score's meaning is totally different. For a high-budget drama with popular actors and ip, reaching 7 500 is average. However, for a low budget with no so well-known actors and not-so-popular ip, reaching 7 500 shows that the drama is a success.“
https://it.mydramalist.com/list/4q6a5k81
Although this show is already a success, 8500 means that we are getting closer to that evasive 9000 mark.