6000Nm and, say, 25 knots (probably a bit on the high side, but not by much) should add 240 hours of steaming or 10 days to the voyage.
(This will be somewhat offset by the lower speed through the channel, waiting times &c - I've no idea how long a typical Suez transit is.)
For yachts it is (supposed to be) two days. It's one day to Ismailia on either way and then one for the rest. (The paperwork however, may take longer, but can be arranged in advance.) See more at https://www.noonsite.com/report/suez-canal-transit-informati...
That's a lot of fetch for wind, which transfers energy to the ocean's surface, which travels as waves, which build up because there's no land damping from west to east. Throw in the Antarctic Circumpolar current and weather systems interacting with the Benguela current an Agulhas current... That's a lot of energy getting put into a relatively tiny band of water.
The Cape of Good Hope was previously known as the Cape of Storms based on the aforementioned confluence of forces generally making sea states miserable. The Good Hope part was what putatively happened after you got through it, and the seas you'd have to worry about would generally be calmer.