Bug#1125313: libc6: Performance regression in KDE/Qt apps after update to 2.41-12+deb13u1

Package: libc6
Version: 2.41-12+deb13u1
Severity: important
X-Debbugs-Cc: [email protected]

Dear Maintainer,

After upgrading to glibc 2.41-12+deb13u1 (Debian 13.3 point release on 
January 10, 2026), all KDE/Qt applications experience severe startup 
delays of 1-3 seconds. Previously these applications started instantly.

What led up to the situation:
System was updated with 'apt upgrade' which included the glibc update 
from the 13.3 point release.

What exactly did you do:
Tested multiple terminals and applications:
- Konsole: 2-3 second startup delay
- Alacritty: 1-2 second startup delay  
- Other Qt/KDE applications show similar slowdown
- Non-Qt applications (xterm) start instantly

What was the outcome:
All Qt/KDE applications now have significant startup delays making the 
system feel sluggish and unresponsive.

What outcome did you expect:
Applications should start instantly as they did before the update.

Additional information:
- Tested on both kernel 6.12.57 and 6.12.63 - issue persists on both
- GLIBC_TUNABLES=glibc.rtld.execstack=1 workaround does NOT resolve the issue
- Intel Iris Xe Graphics (amd64), KDE Plasma on Wayland
- This represents a significant usability regression for a stable release

-- System Information:
Debian Release: 13.3
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)
Kernel: Linux 6.12.63+deb13-amd64 (SMP w/8 CPU threads; PREEMPT)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages libc6 depends on:
ii  libgcc-s1  14.2.0-19

Versions of packages libc6 recommends:
ii  libidn2-0  2.3.8-2

Versions of packages libc6 suggests:
ii  debconf [debconf-2.0]  1.5.91
pn  glibc-doc              <none>
ii  libc-l10n              2.41-12+deb13u1
pn  libnss-nis             <none>
pn  libnss-nisplus         <none>
ii  locales                2.41-12+deb13u1

-- debconf information excluded