SMTL=Slow Metal
When sparked, spark will be slow.
You slow the spark by cooling it down, this is useless.
Spark doesn't slow down at low temperatures. It either conducts at the same speed it does at room temperature, or it doesn't conduct at all. The only exception is QRTZ, which conducts at the same speed as METL, but only at low temperatures. For a slow spark speed, WATR will work, provided that temperatures and pressures aren't too extreme.
EDIT: There is the DLAY element, which simply holds a spark for a while if it is at a high temperature.
What's the problem with DLAY? It's a nice element to control SPRK.Just adjust it's temp to suit your need.