Today I have a peformance issue where setting legacy cardinality estimator ON helps.
From 100s to 1 sec. Really interesting.
If you have individual queries, and you can edit them, it is possible to add a hint to use the legacy estimator. I shy away from recommending this stuff because usually, the query needs editing anyway, so if we can modify the code, why not try to generally improve it rather than prop it up with a hint. However, it's a good thing to know.