There are several things that might be going on here. First of all, depending on the PDK, you may find that calling one of the callbacks is sufficient (at the end) if the callback is common to all and ensures consistency of all the parameters. Secondly the slowness might be due to the parameter changes causing re-evaluation of the pcell, including intermediate steps as each parameter is set.
If you look at the code referenced in this recent post, you'll see that abInvokeInstCdfCallbacks takes an argument ?order, so you can tell it which callbacks to call. So for example ?order list("w" "nf") would call just those two in that order.
If you want to try to minimize the re-evaluation of the pcell, then I would do something like this:
Then make your changes to cdf - so cdf~>w~>value="2u" or whatever your code needs to do to update it. For example:
cdf~>w~>value=sprintf(nil "%g" w)
cdf~>wf~>value=sprintf(nil "%g" w*nfingr)
abInvokeObjCdfCallbacks(cdf ?order '("w")) ; or whatever order you need
And finally apply these to the instance:
I haven't checked recently (and this probably depends on the version you're using), but I think dbReplacePropList avoids multiple pcell evaluation.
Note I haven't exactly used the above before - I've used similar within a hierarchical pcell though.