Unfortunately I can't test it in v8.5 as I've fully switched to v9.0. Besides the same project can't be tested as data format is not backward-compatible. Does v9.0 do any additional checks (data integrity etc) compared to v8.5?
V9 has new functionality including rule creation, consequentially it may well have to check more rules, if engaged, however it is not expected that a design from V8.5 when opened in V9 would show any marked increase in time to run the same set of selected checks.