How many rows are you working with? I’ve seen it slow before on someone else’s app, but they were searching through thousands of rows. That list had to reload each time a letter was typed and the more letters that were typed allowed the list to load faster because the list got smaller and smaller.
Having a lot of rows can mean a lot of data to cache on the device.
The search will typically search through any visible data in the list as well as any visible data in the details screen of the list items.