When Yahoo Mail seems to be running slow, or certain buttons or features aren't working right, it's often due to the web browser it's being accessed from rather than the account itself. This can include: mailboxes loading slowly, attachments not downloading, buttons not working/not showing, etc.
The quickest way to resolve these issues is to make sure you're using a supported web browser and then clear the cache if needed. Below is a list of supported browsers. Click on the one you use and follow the steps to clear the cache.
Edge (Note that Edge can work inconsistently and one of the above browsers may be a better option)
It's important when clearing the cache to select only the Cache checkbox (if your browser provides this) and set the timerange to clear as far back as the browser allows. This ensures that the correct data is cleared. Also, it's better to clear the cache manually from the browser itself rather than using a 3rd-party program or having it clear on exit.
Cache is a collection of data stored by a web browser on computer for quicker access in the future. Over time, this data can become corrupted and cause web sites to load improperly. Clearing it every so often forces the browser to download new cache and resolve any issues. Keep in mind that clearing the browser's history is different than clearing the cache. Clearing the history won't affect performance, as the history is just a list of websites you've visited.
If you're still seeing trouble, try the following additional steps: