Speed is more of a issue, if you fast walk 5 km/h you'll have 7.5 in 90 mins. I'm sure Rakitic number are also good in that depertment but we need speed and dynamic and Pjanic lacks exactly that.
I've just replied on his post that Pjanic doesn't run which is not true. He didn't mention Pjanic's speed.
And even in speed department he's an improvement over Busi, Raki and Arthur.
Also, if he'll be paired in midfield with FDJ and Puig/VDB/Pedri and we'll also play a more mobile player (even if it's Griezmann who is not really fast) than Suarez alongside Messi upfront we'll have a much more dynamic team we currently have.