Why does my 2010 Excel workbook run slow unless I turn it to manually calculate cells?

It has one short macro, is saved as .xlsm, and has about 20 worksheets (about 6MB of data total). I have used other, larger workbooks in the past with no issues on this same computer. It will freeze up for a second every time I enter something new in a new cell, EVEN if there are no cells linked to it/what I'm entering doesn't change any other cells (it even does it for formatting?).