Huh. Yeah, it is noticeably faster on my 9a with that disabled. I wonder what they're doing differently...
OSM-rendering apps are a fair bit more computationally-expensive than many apps, so I do expect them to show the allocator's cost more, but OsmAnd stands out quite starkly against every other app on my phone.
GrapheneOS can provide a performance vs. security toggle for hardened_malloc for these rare cases but it rarely comes up as relevant.