For performance, avoid any springboard add-ons that are not essential. Most jailbreak apps are just like most app store apps, they only run when you open them. But springboard add-ons run constantly, and especially those with visual components (winterboard the biggest offender, but also 5 icon dock/row, reflective dock, etc) can really slow you down. My phone is totally loaded with this stuff because I can live with the hit in speed most times, and use Swap Mode to disable all of those things when I want the speed back. But when I re-jailbreak (after the next firmware update) I will probably just add SB Settings as my only springboard mod. It's the only one I can't live without, and by itself it shouldn't slow me down too much. The rest of the JB apps I use are just apps, not SB add-ons.
I will miss my theme.... But the speed is just.. well... better.