Performance is always great until you have to hit disk. Not uncommon to rely on mmap at which point your disk access is sub-optimal vs. a hand-tailored buffer manager with strategies to improve disk reads.
the purpose of a btree is to optimize when you are hitting the disk, you can't call that the struggle, that's when the btree sings (tho you could consider extensible hashing)
Performance is always great until you have to hit disk. Not uncommon to rely on mmap at which point your disk access is sub-optimal vs. a hand-tailored buffer manager with strategies to improve disk reads.