Joel thinks so; David Heinemeier Hansson has a great pragmatic answer; Smalltalk guru Avi Bryant explains why dynamic, duck-typed languages don’t have to be slow at all, both he and Patrick Logan point out the link to Hotspot’s Smalltalk predecessor, StrongTalk; Steve Vinoski (who certainly has done his share of C++ development) criticizes Joel’s “fundamental misunderstanding”; Obie is disappointed by Joel.
I still think Joel is one of the greatest writers on the ‘net, but in this case, I agree he’s plainly wrong.
So Joel is plainly wrong when he says that Ruby is slow? Have you seen http://shootout.alioth.debian.org/?