Prepared updates (with USE KEYS) are 37 slower than standard in line N1QL

I am not positive but I think this only slows down or impacts the USE KEYS prepared statement use case.

My tests (I can provide a complete test setup one shot script if desired)

Across 35M documents I get can process 12203.62 items/sec 50% deletes and 50% update a TTL as follows:

var stmt = "UPDATE `wu_src` USE KEYS \"" + key + "\" SET meta().expiration = " + Math.floor(ttl / 1000);

N1QL(stmt);

However across 35M documents I only process 327.39 items/sec with the same mix  50% deletes and 50% update a TTL as follows:

var set_ttl = Math.floor(ttl / 1000 );          

var stmt = "UPDATE `wu_src` USE KEYS \"" + key + "\" SET meta().expiration = $1";

N1QL(stmt,[set_ttl], { isPrepared: true });

FYI it doesn't matter if I pass {"isPrepared": true } I get the same degenerate performance.

Why the 37X slowdown? Is this an optimizer issue?  I would have thought the the later prepared statement would have been faster.

A bit of good news is that in 6.6.1 (Advanced Bucket Ops specifically 6.6.1-9212-1) we can do the same (avoiding N1QL entirely) customer process without N1QL as follows at 29659.66 items/sec (same 50% delete 50% TTL update mix)

var res = couchbase.replace(src,{"id":meta.id,"expiry_date":new Date(ttl)}, doc);

but of course this doesn't help 6.6.0 users or other UPDATE type uses of N1QL(...) in TBD use cases.  

FYI just doing 50% deletes (no TTL updates) which have no performs issues gives me 52083.33 items/sec on my setup.