Short answer

• Updates can either improve or hurt perceived speed depending on what they change. Many updates optimize system code and drivers so things run faster or more reliably; others add features or background services that use more CPU, memory, or storage and can make the device feel slower — at least until the system settles.

Why updates affect speed (key mechanisms)

• Code changes: Security and performance patches can make the kernel, file system, or apps faster (or occasionally slower if new features add overhead).
• Drivers and firmware: Updated GPU/SSD/network drivers or firmware often improve throughput and latency; bad or incompatible drivers can reduce performance.
• Background work after update: Indexing, reindexing search databases, app migrations, thumbnail generation, or system optimization tasks run after updates and temporarily use CPU, disk, and battery.
• New services/features: OS features (telemetry, cloud sync, visual effects, security scanning) can increase background CPU/memory usage.
• Storage use: Updates consume free space; when free space is low the system can slow down (swap/virtual memory, file fragmentation on some systems).
• App compatibility: Older apps may not be optimized for the new OS, so they can run slower or use more resources.
• Thermal and power management: Different power/thermal policies in an update can change CPU boost behavior, which affects peak performance and battery life.
• Firmware/driver regressions: Rarely, an update can introduce inefficient code or bugs that degrade performance until patched.

Typical short-term vs long-term effects

• Short-term (hours to a few days after update): Temporary slowdown is common because of background tasks (indexing, optimization, updates to apps). System may feel sluggish during this window.
• Long-term (weeks/months): Performance usually stabilizes. If update included optimizations, you may see sustained improvements. If it introduced heavier services or regressions, the device can remain slower until fixed.

How it differs by device type

• Windows PCs: Updates often include drivers, security scanning, and feature changes. Low-RAM or near-full-disk machines are more vulnerable to slowdowns. Windows will perform background optimization (e.g., app indexing, Windows Search/Defender activity).
• macOS: System updates can change spotlight indexing, background daemons, or drivers. APFS/SSD optimizations usually help performance. Low-storage Macs or older hardware can be impacted.
• iPhone/iPad (iOS/iPadOS): Apple tends to optimize tightly. Updates may temporarily use more battery and CPU for indexing (Photos, Siri). Older devices can lose some peak performance intentionally (power management) or due to heavier features.
• Android: Varies widely by vendor. Google updates and vendor firmware/driver updates can improve performance, but manufacturer skins may add background services. App compatibility across Android versions is a factor.
• Linux: Kernel updates and driver updates can improve throughput/latency. However, distribution upgrades may change background services or desktop environments that affect responsiveness.

Practical signs an update affected speed

• High CPU or disk activity for a long time after update
• System feels laggy or apps take longer to open
• Shorter battery life or increased fan/noise
• Higher memory use and swap activity
• Network slowdown if a new service is syncing more

What to do before and after updates (recommended checklist)

• Before updating:
  • Backup important data (Time Machine, Windows Backup, cloud, etc.).
  • Free up space: aim for ~10–20% free on HDD/SSD to avoid swap/fragmentation slowdowns.
  • Note current driver/firmware versions if you want to roll back.
• Immediately after updating:
  • Let the device idle for a few hours so indexing/optimization finish.
  • Reboot once or twice if the updater didn’t already.
  • Check storage, battery, and CPU usage.
• If device remains slow:
  • Check Activity Monitor / Task Manager / top/htop to find resource hogs.
  • For Windows: run Windows Update driver checks, Device Manager to roll back a driver, run Disk Cleanup, consider disabling unnecessary startup apps.
  • For macOS: check Spotlight indexing (mds/mds_stores) and let it finish; safe-boot and reset SMC/PRAM if odd behavior persists.
  • For mobile (iOS/Android): check background app refresh, disable/limit sync, and consider reinstalling problematic apps.
  • Update drivers/firmware from the hardware vendor (GPU, SSD).
  • If serious regression, check vendor/OS forums and bug reports for known issues and patches.

When to rollback or seek help

• Consider rolling back if:
  • Performance is significantly worse and you find a clear cause (e.g., new driver).
  • The update causes system instability, crashes, or data loss.
• Seek help if:
  • You can’t identify the cause and performance is unacceptable.
  • Vendor has a known patch or guidance (check support pages/forums).

Quick commands/tools to diagnose slowdowns

• Windows: Task Manager (Ctrl+Shift+Esc), Resource Monitor, Reliability Monitor, DISM / SFC scans.
• macOS: Activity Monitor, Console.app (logs), sudo fs_usage, reboot in Safe Mode.
• Linux: top/htop, iotop, dmesg, journalctl, systemctl to inspect services.
• Android: Developer Options -> Running services / Battery usage.
• iOS: Settings -> Battery / Background App Refresh; use Analytics logs if needed.

Final tips

• Keep updates enabled for security, but on very old hardware consider testing major upgrades first or deferring feature updates until they are proven stable.
• Maintain healthy free disk space and keep device drivers/firmware updated from vendor sources.
• If an update causes long-term slowdown, check for subsequent patches — vendors commonly release fixes when performance regressions are reported.

If you tell me your device type and the OS version you updated to, I can give targeted checks and step-by-step commands to diagnose and fix the slowdown.