Attempt to split big table into smaller one made query slower

[Patrick St.Onge]

I'm trying to increase the performance of my database by splitting a big table into smaller ones. the big table has data for many clients so I duplicated it and deleted all the data except for one client. This new table has 1 million rows instead of 20 million. I ran optimize table on it to get the size down. I was hoping this would speed up my query, but instead, it made it slower. How could a table with a lot less rows be slower? I tried deleting any irrelevant indexes and columns (like the clientID) but it made no difference.