Just adding in that I have the same exact issue (Houdini 20.5.332), and am hoping that there is a solution, as I would prefer using the service then the sharedserver.
Cleared this up with support - turns out the service is not supposed to work with flow, formerly shotgrid. The shared server method is still the way, despite what the docs say.
The shared server method if I'm not mistaken spawns one actual server per input task, and processes then the queries for each input task, which is not really optimized for Shotgrid (Flow, whatever ^^)
At least that is the conclusion I have when I see Nx3 tasks spawning at the begin server block for N input tasks.
I was hoping that the service would allow us to process requests from 1 single connection point and avoid flooding the Shotgrid server, as I often have hundreds of inputs.
Maybe it helps that they are in a block, and executed in a batch, so there is no new server start for each request. So far it's taking a couple seconds for the usual operations we do - could be better but not a huge deal.