C# intelliSense intermittently failing/very slow when C# extension is running

## Environment data
C# extension version 2.87.31

.NET SDK:
Version: 9.0.304
Commit: f12f5f689e
Workload version: 9.0.300-manifests.ad61bb1c
MSBuild version: 17.14.16+5d8159c5f

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.304\

.NET workloads installed:
There are no installed workloads to display.
Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.8
Architecture: x64
Commit: aae90fa090

.NET SDKs installed:
6.0.428 [C:\Program Files\dotnet\sdk]
8.0.413 [C:\Program Files\dotnet\sdk]
9.0.304 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.19 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

## C# logs
2025-09-08 08:34:19.452 [trace] Activating C# Extension
2025-09-08 08:34:19.452 [trace] C# Extension activated in 115.079ms.

## Steps to reproduce

1. Open VSCode
2. Wait for extensions to fully load
3. In a piece of C# code type 'Console.Wri'

## Expected behavior
- Dropdown list with 'WriteLine' to appear

## Actual behavior
- Dropdown list does not appear
- 'Loading' tooltip appears when I hover over 'Console.'

## Additional context
- Occasionally IntelliSense does work with C# extension running (particularly if I have installed and reinstalled it). However eventually it stops working again in the VSCode session
- Worked OK until last few weeks. I have switched from a laptop running Windows 10 to one using Windows 11 in that time, so maybe that is linked
- If I disable the C# extension IntelliSense works fine
- I am running the C# extension together with OmniSharp