Hey there, let's break down your questions step by step for your 2015 MacBook Pro:

What does the red marker in Activity Monitor mean? #

That red label next to Flash Player means the process is Not Responding—it's completely stuck, unable to process system commands, exit properly, or stop consuming resources. This is exactly why it's still hogging CPU even after you've closed the Safari tab with Flash content.

Why is Flash using so much CPU? #

1. When opening Flash-containing pages #

Flash was notoriously inefficient, especially on older hardware like your 2015 MBP. Here's why it spikes CPU:

• Flash was never optimized well for macOS, and its plugin architecture adds extra overhead when running in Safari.
• Many Flash-based content (like old ads, games, or unoptimized video players) force your CPU to work overtime to render their heavy, outdated code.
• Your 2015 MBP's CPU isn't as powerful as modern models, so it struggles more with resource-heavy Flash content compared to newer machines.

2. After closing the Safari tab #

This is a classic, long-standing Flash bug: Safari's Flash plugin often fails to terminate its background process when you close the tab. Instead, the process gets stuck (hence the red "Not Responding" marker) and keeps eating up CPU resources indefinitely. Flash had a terrible track record of poor process management on macOS, especially with Safari.

Fixes to try #

• Force quit the stuck process: In Activity Monitor, select the red Flash Player entry, click the ✗ icon in the top-left corner, then choose "Force Quit". This will immediately free up your CPU.
• Limit Flash usage: Open Safari > Preferences > Websites > Plug-ins, set Flash to "Ask" or "Off". Only enable it when you absolutely need to use a Flash-only site.
• Note on updates: Adobe ended support for Flash in 2020, so no official updates are available anymore. If your Flash plugin is extremely outdated, the last few versions might have minor bug fixes—but keep in mind, Flash is no longer secure to use.
• Use HTML5 alternatives: Most content that used Flash has been migrated to HTML5, which is far more efficient and secure. Check if the site offers an HTML5 version of the content you're trying to access.

内容的提问来源于stack exchange，提问作者aircraft