Question: advice on using qual in my foreign data wrapper

[frbsfri]

I wrote a foreign data wrapper for HDF5 files, I can perform any query on data. However, some queries are really slow.
for example this query takes Time: 12388,885 ms:

select "printerId", date,"ZPosition" from myforeigntable where ullid=54077217821;

printerId   |   date   | ZPosition
--------------+----------+------------------
FER921002231 | 20130102 |   -6
(1 row)

How should I write quals in my code? please give me an example of using quals and a good description.

Thanks,