If you are in the habit of not cleaning your email resulting in a massive amount of data, you will begin to cause problems with the mail program. Searches will take longer, performance may slow and eventually the data size will get so large that you will stop receiving mail altogether.
To resolve this issue consider archiving your mail. In Outlook this practice is used to shrink the size of very large .pst files to resolve many issues with the software. In Gmail, the practice of archiving has not been used as much because there are size limits to every mail account. Once you reach that limit the mail stops arriving until you delete some. This forces the end user to maintain the account. If you want to keep more mail in the same amount of space allocated, archive the data by date.
Pick a date you can use to archive mail. For example, selecting 2016/12/31 will archive all mail before that date. In other words, all your 2017 mail will stay where it is and everything prior will be in the archive folder. To do this in Gmail, open your account. Enter in the Search bar at the top before:2016/12/31. Click to Search. Once all the mail is found, click the box to select all. Then click the line in the center top to select all the messages found. Now every message is checked. Click the folder with the down pointing arrow to archive.
Once completed you mail will be in the archived folder that may be hidden. When you search for an email the function will include mail within the newly created archived folder. To see archived email, search for the term archive and they will be listed.