You definately don’t need suspension… I’d take my rigid round any regular trail I’ve ridden on the bigger bikes, it was great fun at innerleithen on the black and downhill trails frinstance. Wouldn’t try it on harder dh trails and it’s not a lot of fun on really lumpy routes- I wouldn’t take it round the beast of brenin, I know it’d do it but I doubt I’d enjoy it much.
Slower? Aye, unless it’s very smooth. And I mean very smooth. You’ll hear people moaning on here about how Glentress is too smooth frinstance, it’s just a sanitised gravel path, all whacker-plated to death… Try it at speed on a rigid, you find where those bumps went, your suspension soaked them up and you forgot they existed.
But on a lot of trails the difference is less than I’d expected, and a good rider can still drop a bad rider. Plus if you’ve got all your time on rigids it takes time to learn how to make real use of suspension, took me ages to learn to let the fork work.