auto filter very slow to return results

i have a work book of 12 sheets , the two main sheets 1+2 have 5000+ rows.
by 30 columns
each row column b is a site number in ascending order(up to 4 for each site) so auto filter select = to site 5
about 10 seconds later i get the result of 3 sites
try site 846 30 odd seconds later up pops the result. so i copied the sheet to a new work book and tried and it's nigh on instantaneous.
next step
copied sheet paste special values into new sheet in same original workbook
and tried filter on that still slow as ever,
thing is this book around 8meg and i cant for the life of me work out why
ive cleared excessive formats and reset last cell on each sheet.
there is a vlookup from sheet 1 to 2 for each site to return serial numbers.
i have tried it in vba by turning autocal off/on but still no difference.
any ideas if anything else could cause this?