Hm I did a test with an coverage-instrumented Python binary, and you are right. I counted the number of times lookdict_string in dictobject.c is called -- call

Hm I did a test with an coverage-instrumented Python binary, and you are right. I counted the number of times lookdict_string in dictobject.c is called -- call that number D. And then I made N attribute accesses like 'p.x'. Without slots, if N goes up, then D goes up. With slots, as N goes up, D stays constant. Thanks for the information -- I'll look into this more. (However, as mentioned, slots are not more than 20-30% faster on my machine, in this limited benchmark. I feel like this is a double mystery. Or maybe it's just that interpreter loop overhead or some other overhead is the bottleneck, and not dict access.)