[RF] Building of Histograms/Errors from a workspace with a region with many bins is super slow

[TomasDado]

Explain what you would like to see improved and how.

If one builds a HistFactory workspace where one region has many bins (e.g. 1000 bins) and then wants to make a plot with the proper error band (even pre-fit) this takes a very long time (on my machine its an order of 20 minutes). This is almost instant with 10 bins or so.

example code:

#include "RooFit/xRooFit/xRooFit.h"
using namespace ROOT::Experimental::XRooFit;

void mwe() {

  RooRealVar::enableSilentClipping();

  xRooNode ws("ws.root");
  auto pdf = ws["simPdf"];
  auto channel = pdf->find("ProblematicRegion");

  auto errors = channel->GetBinErrors();
}

With the WS that can eb found here: /afs/cern.ch/user/t/tdado/public/reproducers/ws.root

This is the screenshot from the profiling of the code:

ROOT version

ROOT Version: 6.39.01
Built for linuxx8664gcc on Jan 29 2026, 07:58:17
From heads/master@v6-39-01-856-gee341853130

Installation method

built from source

Operating system

Ubuntu 25.10

Additional context

No response