I do not see anywhere documented this, the more Access policies degrade performance for sure - because it needs to process top-down manner.
Optimize always Access policies for a better match and keep housekeeping when required remove redundant access policies.
Keep monitor always CPU and performance, how the requests are processing - based in the response time. ( by configuring access logs with syntax give you any delay in processing the requests.
for reference guide: based in 11.X version
https://www.cisco.com/c/en/us/td/docs/security/wsa/wsa11-0/user_guide/b_WSA_UserGuide/b_WSA_UserGuide_chapter_010010.html