I want to get a SSD for my Lenovo Yoga 2 11. Current drive is a slim line WD (with a proprietary connector which will cost a relatively-small fortune to get around) which is also very s-l-o-w.

I see most SSDs have fast read speed but some have write speeds about 40% or so read speed (others have write speeds near the same). How can one determine how fast a computer can actually write at? Don't want to invest too much if it isn't going to be beneficial.