High ID (unique, auto increment) causes slow responses on PHP (MySQL)

I've built a certain system where every time new user enters it creates a "guest row" on "guests" table. he also gets an identifying cookie. The table contains several fields, one of them is ID which is auto increment and unique When he does a certain action it gets recorded in the "done_action" table with several fields on it, one of them is "guest_id" and his ID on it. Problem is the guests table has about 30,000 rows (which means ID is high). I was wondering whether I should somehow change how the system works. I've thought about couple of options A weekly "cleanup" job. opens a table called "guests_tmp", *copies* all data from guests to guests_tmp. every row it copies it also changes the ID on the table done_action. What do you think? -- Use ROT26 for best security