SPI Concurrency Precautions?

Dear Hackers

I've been working on an extension and using SPI to execute some queries.
I am in a situation where I have the option to issue multiple queries
concurrently, ideally under same snapshot and transaction. In short, I
am achieving this by creating multiple dynamic background workers, each
one of them executing a query at the same time using
SPI_execute(sql_string, ...). To be more precise, sometimes I am also
opting to issue a 'CREATE TABLE AS <sql_query>' command, an SPI utility
command.

I was however wondering whether I can indeed achieve concurrency in this
way. My initial results are not showing much difference compared to a
not concurrent implementation. If there would be a large lock somewhere
in SPI implementation obviously this can be counterintuitive. What would
be the precautions I would need to consider when working with SPI in
this manner?

Thanks,
Tom