I doubt you could narrow it down to any particular coins wallet, as it seems it can be any wallet at any time that might either respond immediately or take a long time to return. I assume the wallet must be doing something that prevents it from processing rpc calls at the time.
Calling GBT with no parameters can return a massive amount of data on the BTC wallet. Do you know if it is making a GBT call on BTC? Since this is not being mined directly, it might be possible to put an exclusion in for the BTC wallet.
Yeah I can't really. I tried reducing them all to just main stream, higher diff coins and it would still happen. Stratum has to be always calling GBT on all coins all the time after each block notify. BTC isn't enabled in stratum so those calls don't get made but yes, BTC GBT spits out a lot of crap. Early on it was enabled and saw errors all the time but isn't the source of the on going issue unfortunately.