Why PD disable radix cache and page size =1 get better performance for nixl backend?

[llc-kc]

Why PD disable radix cache and page size =1 get better performance for nixl backend?
for radix cache, prefix cache should reduce prefill compute, but results in performance drop.
for page size, I think large page size should be helpful for transfer large block of data, but results in performance drop.

[fungaren]

#3554 May be relavant