Hi Gideon
I've seen a situation in which B1's client was blocked by B1iF but it was certainly due to database locks - I was executing some DML statements...
Maybe there is something that does anything against B1's db?? Dashboards??
If you had a chance (I presume that it's possible since you wrote that you disabled B1iF service) I'd recommend to make another test and deactivate all the scenarios and check behavior of the client. If it's better you can activate scenarios one by one and check if it's getting worse.
Kind regards,
Radek