Yes. It is a Microsoft-signed optional feature that runs in parallel with newer versions (4.x). It does not slow down your PC.
Starting with Windows 10, the only fully supported way to install .NET Framework 3.5 offline is by using the found on the Windows 10 installation media. 1. Prepare Your Media net framework 3.5 download for windows 10 offline installer
Download the official Windows 10 ISO from Microsoft using the Media Creation Tool. Step 2: Mount the ISO by double-clicking it. Note the drive letter (e.g., D: ). Step 3: Open Command Prompt as Administrator (Win + X → Terminal (Admin) or CMD Admin). Step 4: Run the following DISM command (Deployment Imaging Service and Management Tool): net framework 3.5 download for windows 10 offline installer