[bitcoin-dev] Why SegWit Anyway?

[CANNON]

On 11/21/2017 01:16 PM, Adán Sánchez de Pedro Crespo via bitcoin-dev wrote:
> 2. SegWit signatures can be cheaper to verify (linear instead of
> quadratic). Prior to this, DoS attacks were possible by using forged
> transactions including signatures which could take several minutes to
> verify.

Where can I find more resources on this described DoS attack?
And how does SegWit prevent this if using SegWit transactions are not enforced?

Thanks