Check whether the tstart command is issued with a post conditional parameter if so then you may be able to make the ts fail and therefore the ts would not start the transaction.
For instace if you had:
Tstart:$g(^startts)=1
Then killing ^startts would cause the transaction not to be created.
#longshot
Have you tried looking at the query plan for your query to see which indexes it is using and whether these appear to make sense?
It appears that the generated query will often ignore the sessionId index as below:
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Look at the parameters you can pass to the use command - the below will do what you want
u $i:(:"+S")
read password
or if doing directly at the command line instead of from a called routine
u $i:(:"+S") read password