As far as I understand, GrapheneOS's requirements focus on security features, not device speed.

It's specifically the ability to relock the bootloader with custom software. Pixels are the only phones that can do it.

Doesn't seems to mention spec, the announcement article did mention a lot of security and it's a joint venture to seek out better security for your security of your secure phone.

They need several years of updates for firmware, baseband radios etc from the OEM, They also need a robust secure element, this is why spec requirements are as they are. Mainly for the end user's security & privacy can't have privacy without actual security.