By "Shylock's speech," I'm assuming that you are referring to Shylock's "Hath not a Jew eyes?" speech in Act 3, Scene 1, which is probably the moneylender's most famous speech (and perhaps the most famous speech in the whole play). If you're unfamiliar with this speech, here is the most important part of it:
... He hath
disgrac'd me and hind'red me half a million;
laugh'd at my losses, mock'd at my gains,
scorned my nation, thwarted my bargains,
cooled my friends, heated mine enemies. And
what's his reason? I am a Jew. Hath not a Jew
eyes? Hath not a Jew hands, organs, dimensions,
senses, affections, passions, fed with the same
food, hurt with the same weapons, subject to
the same diseases, healed by the same means,
warmed and cooled by the same winter and
summer, as a Christian is? If you prick us, do we
not bleed? (46-58)
In this speech, Shylock describes the abuse he's suffered at the hands of Antonio, and points out that this oppression is a result of the fact that he's a Jew. More importantly, Shylock questions why he should receive such ill-treatment, as a Jew is a human being just as a Christian is. This speech is important because it uncovers the anti-Semitism at the heart of the play and points out its hypocrisy. More to the point, it reveals that Antonio is not as virtuous as he seems, but is actually something of a racist. As such, this speech is appealing because Shylock is proving that he is not just a "villain"; instead, he is an oppressed individual who suffers greatly at the hands of his Christian oppressors. In that case, even if Shylock can play the villain, this role is complicated, as the Christian community in the play has driven Shylock to evil by subjugating him to cruel treatment and prejudice.
Determining how this speech might be performed is somewhat subjective. That said, I believe that it would have to be performed with a complex mixture of hatred, outrage, and sadness, as Shylock is essentially conveying that he is tired of being treated as sub-human just because he is Jewish, as he is exactly the same as his Christian tormentors. Therefore, any performance of this speech would have to effectively convey the complex and subtle layers of emotion displayed here.