Another possibility ...
1 - You run the proc regularly enough during the day that a compiled copy remains in procedure cache thus eliminating the need to recompile the proc for your regular executions.
2 - You cease running the proc for an extended period of time leading up and including the time during which you're loading data.
3 - The first time you run the proc (after loading the data) the proc has to be read from disk and compiled.
This of course assumes that it takes upwards of 13secs to comiple the proc (quite possible with ASE 15.x, less likely ... though not impossible ... in ASE 12.5.4).
To see if the above describes your situation you can try the following at any time that's convenient for you:
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exec <proc> <argument_list> with recompile
go
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This will force the proc to recompile the next time it's run. If your 13sec delay (after the data load) is due to having to compile the proc then this call example should also take ~13secs to complete.