You may be experiencing any of these issues:

• Applications take a longer than expected time to open.
• Your phone takes longer than expected time to complete actions (such as swiping screens, or navigating settings).
• Keyboard or other inputs are delayed before they appear on screen.
• Your phone generally performs slower than expected.

If the internal or SD card storage of the phone becomes full, it may cause certain actions that use or access the storage to perform slowly. Follow these steps to uninstall large or unused apps, and remove unused files:

1. Go to Settings > Apps > Downloaded and tap the application, then tap Uninstall.
2. Search for and delete unused files, by going to Settings > Storage > Make more space.

Some issues can be resolved by power cycling your phone. Follow these steps:

1. Press and hold POWER until the Phone Options appear.
2. Select Power Off and wait for the device to fully turn off.
3. Wait for a few moments, and then power the phone on by pressing and holding the POWER button.

Ensure the phone is fully updated, by following these steps:

1. Go to Settings and select System Updates. The exact path will differ depending on your phone.
   These are the three typical paths:
   
   Settings > System updates > HTC software update
   Settings > Software update
   Settings > AT&T Software Update
   
   

2. Select HTC Software Update to check for new updates.
   If an update is available, follow the on screen prompts to download and install the update.
   If no update is found, confirm the phone is on the latest software version going to Settings > About > Software Information.

Clearing the data cache may resolve the issue. See How to wipe your cache partition.

A factory data reset can resolve some software issues. See How to factory reset a device.