After installing Lion my mac became super slow
Activity monitor is your friend in this case. Check CPU/RAM usage and look for apps/services using a lot of resources, also check how your hard disk is doing in terms of throughput.

I've seen machine's hard disks slow down or just fail after an OS upgrade/install. It's a pretty disk intensive operation and can finish off older disks that were on their way out. Consider that as a possibility - perform a backup as soon as possible if you don't already have a recent one.