This could be because your anti-virus software is checking every file when it is opened. This means that the database table files are scanned once every call.
Also your application may be coded to reinitialise the software each time an API call is made. This incurs a performance penalty.
Re-configure your anti-virus software to prevent it scanning the DTFs on every call.Or modify your calling application to initialise Bank Wizard for Bill Payments at startup, instead of upon each call.