When editing large files (~1000+ lines), Cursor (with Agent enabled) becomes extremely laggy on a Mac Mini M4.
Details:
Opening and editing a file with around 1000+ lines causes severe performance issues
Basic actions such as:
Mouse clicking
Scrolling
Typing
are delayed by 5–10 seconds
The editor frequently freezes and becomes unresponsive for a short period
Each interaction requires waiting before it is executed
Expected Behavior:
Smooth, near real-time response even with large files
No noticeable delay when scrolling, clicking, or typing
Actual Behavior:
5–10 seconds delay per interaction
UI temporarily freezes during user actions
Environment:
Device: Mac Mini M4
App: Cursor (Agent enabled)
File size: ~1000+ lines
Impact:
Severely affects coding experience
Makes continuous work difficult and time-consuming
Steps to Reproduce
Open Cursor on a Mac Mini M4
Ensure Agent is enabled
Open or create a file with ~1000+ lines of code (any language)
Start performing basic actions:
Scroll through the file
Click to move the cursor
Type or edit text
Observe the editor performance
Result:
Each action (scroll, click, typing) is delayed by ~5–10 seconds
The UI becomes temporarily unresponsive after each interaction
Expected Behavior
The editor should remain smooth and responsive even when working with large files (~1000+ lines)
Scrolling, clicking, and typing should happen in real-time without noticeable delay
The UI should not freeze or become unresponsive during interactions
Enabling Agent should not significantly degrade editor performance
Overall coding experience should remain stable and efficient across typical file sizes
Hey, thanks for the detailed report. A 1 000-line file isn’t that big, and a Mac mini M4 should handle it fine, so something’s clearly off.
To narrow it down, we need a bit of diagnostics:
Process Explorer: when the lag starts, open Cmd+Shift+P then run Developer: Open Process Explorer. Send a screenshot so we can see which process is using CPU (extensionHost, renderer, ptyHost, etc).
Test without extensions: try launching Cursor from Terminal with cursor --disable-extensions, then reproduce the issue. This will tell us if it’s extension-related.
Spindump: if you can catch a freeze, open Activity Monitor, select Cursor Helper (Renderer), then click Sample Process. The text output will be helpful.
One more question: what language are the files in, and does the lag happen only with Agent enabled, or also when you just open the file?
The issue happens mainly when Agent is enabled. If I just open the file without using Agent, performance is mostly normal.
File type: mostly PHP / JS files
File size: around 1000–1500 lines
I’ll provide the diagnostics you asked for:
I’ll capture a Process Explorer screenshot when the lag happens to see which process is spiking (extensionHost / renderer / etc.)
I’ll also try running with cursor --disable-extensions to rule out extension-related issues
And I’ll collect a spindump (Sample Process) from Activity Monitor during a freeze
From my observation, the lag feels like the UI thread is being blocked (scroll + typing both freeze), especially when Agent is actively processing or after it modifies the file.
Hey, I can see the Process Explorer screenshot, it’s really informative.
The main thing that stands out is the process window [5] nvmr.php using 155 CPU and 2 802 MB of memory. That’s the renderer process for the window with your PHP file, and numbers like that for a 1 000 to 1 500 line file are definitely abnormal. This also matches what you noticed, the UI thread is getting blocked.
Most likely the issue is in how Agent applies and renders diffs in the file, the renderer gets overwhelmed while processing the changes.
Also worth noting, you have Intelephense running, the PHP language server. That can make things worse when Agent modifies the file and Intelephense tries to reindex it at the same time.
Two next steps to try:
Test with cursor --disable-extensions. This will show whether Intelephense is contributing to the issue, or if it’s purely a renderer or agent problem.
Capture a spindump during the freeze. This will give an exact call stack showing where the renderer is getting stuck.
The team is aware of similar renderer performance issues when Agent works with diffs. Your report with the Process Explorer data helps us prioritize it. Let me know the results of the --disable-extensions test.
