I’m using jqGrid 4.6.0.  I tested in both both IE8 and IE11.  It seemed about the same.  I took columns from 30 to 15.  15 columns and 1000 rows in IE8 is taking around 4.2 seconds.  15 columns and 500 rows is taking about 2.2 seconds.  I probably have some flexibility with business requirements on the page size, but reducing the number of columns will be a harder sell.