Sparsebundle on USB Key is very slow

I have an 120 GB encrypted sparsebundle on a USB Key. I access it via a recent mac book pro with latest os. The USB Key is a good brand, USB 3, 128 GB.

When mounting the sparsebundle from the USB Key, access to it is extremely slow.

Is it expected ? Can I do something about it (change file system ? Not use sparsebundle, but encrypt key instead - if that's possible) ?

Thanks !
Sparsebundle on USB Key is very slow
 
 
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