pdf files load slowly

I have been having a problem loading pdf files ; they seem to load a line at a time. I have been using the Sumatra reader. I also tried a FoxIt reader with the same results. Every now and then ,I get a message telling me that the pdf reader is not the best and I should get a better one but it seems the reader is functioning in that respect.

Is the slowness a function of the reader software or the computer?

I also have a netbook with windows 7 starter and I used the same pdf readers and it functions fast.

I did not download adobe because it seems to take up a lot of room with daily alerts to upgrade. I just want to read a pdf file.