I don't even consider Arthur slow either.
Rakitic on the other hand runs like he's being weighed down by sandbags. He may not be as slow as Cesc but he's still slow as fuck. The thing is you if your reading of the game is good enough and your ability to retain possession and being good with the ball under pressure can go a long way in help migrate some of the flaws like the lack of pace.
With Rakitic my real opinion on him is pretty simple. I see him as a guy who's intelligent and especially in a two man Midfield he can ping some nice balls here and there and a lot of teams would kill to have him on their teams.
However his biggests weaknesses is in a possession system in a three man Midfield his flaws are more apparent. He's not good in tight spaces, He's not great handling being pressed with the ball and it seems unless the conditions are right that he struggles to attempt creative passes in a RCM. He either doesn't have the vision for it or he's feels not confident in his own mobility and pace to help track back if the ball ends up being intercepted and the team counters.
In the second half of the 14/15 season and for a good portion of the 15/16 season He did well as he had Alves and Messi on the right hand side to help him out in terms of creativity and it allowed him to not worry as much either and every now and then he did contribute some nice creative passing.
After Alves left and Messi not being all the interested in staying as a RW. He did form a pretty decent connection with Bob but Bob isn't Alves and the RW was just vacant leaving the guy and Bob without anyone to linkup with and in general his 16/17 season was meh for the most part. And there was a time where Lucho had trusted even Gomes ahead of him until Iniesta got that injury vs Valencia.
He didn't play all that well at first under Valverde. It wasn't until the switch to the 4-4-2 is when he began to regain form. His role seemed pretty simple, help maintain control with Busi and try to sniff out attacks as best he could and it pretty much worked although in the CL he like the rest of the team wasn't all that great though.
He was a steal for his price and perhaps one of the best deals we ever made even though he has a number of limitations.