I went to the site and downloaded the full client, first with Firefox 7.0.1 and then with GetRight download manager. Both downloads were the same size and at approximately the same download speed. They were not 643 MB. Perhaps I downloaded a different file.
DFI_SFSingapore_150911.exe
Size: 629120599 bytes
Modified: Sunday, October 30, 2011, 11:38:17 AM
MD5: C946B378CE68FC35EF74E6B0ADA6AF4A
SHA1: A19D3D8255BA2CBC44A675285595B615AC95B3EC
CRC32: 5F3208DD
Like you, in Firefox the download paused slightly while Avast scanned the partial download at 19.4 MB for me, then proceeded to completion. With GetRight, although I did see some Avast activity during download much sooner there was no delay in download. I have GetRight set to use AshQuick.exe to scan the download upon completion.
I see no problem with this.