you could also use a SATA-USB enclosure to connect to pc(I use an IDE-USB enclosure for my 250 gig western Digital since its IDE, ahd to mod my network adapter though).
About reading speed: You Will indeed have some lag when starting hdloader(i guess it depends on how big your hd is and how full it is), but it should be bearable(i get about 20s lag on my hd, but its almost completely full). gameplay speed is not affected, and load times decrease significantly compared to playing on cd/dvd. OH, and playing via hdd does not affect dvd playback since you are not using the dvd drive at all
V7 ps2, flip top, 2x 8MB sony memcards, FMCB 1.8b, ESR r9b, Cogswap, Hdloader0.8c, 250 gig Western Digital, poorly modded(but functional) network adapter