Adding z field will not be any problem, could be issue with table update statistics or storage index quality issue.
do the following sequence
Get the trace and see the CPU cost, if its in red then problem with your query, and may be wrong index selection.
Execute the report RS*QN and see the Index storage quality(rebuild the index if its less than 60%)
Activate the table histogram.
Still your issues persist the contact BASIS to ask re-org the table.
Regards,
Vadamalai A
