I think McTominay and speed is one of the least important things for a passing midfielder
Speed or more importantly acceleration over the first three, 4 meters and dynamism is very important aspect of midfielders game. That's where you win important duels without being tall and strong, that's how you cover for your teammates when they lose the ball, very important pressing-wise and forcing other team to make a mistake.
Going forward super important to be able to quickly drop off and make yourself available for a pass and angle for a quick one two, or just to have a space and time to get take a good pass yourself.
That's why we struggle a lot with Matic and McTominay more often than not. With Pogba it can be also very tough but he showed to have some excellent world class games from deep too being just superb passer and playing with incredible vision and ball covering when he's not too lazy and have an easy escape. However he needs legs and himself be fit and motivated.
With Garner I don't think we're getting a fast player, certainly not but he's got that good acceleration over the first couple of meters and if he builds his stamina and strength further we'll have very good midfielder playing at pivot providing a lot of things I mentioned to be crucial to be playing there.
He will probably end up being as fast as Herrera if we directly compare them in the end but not even for Ander it was that important to be fast nor that strong to match Kante's defensive statistics in interception/tackles in a season or two.
Think in current set up he would be easily best paired with Fred who would provide immense workrate and and overall great defensive contribution along with some quick short passing bewteen the lines while Garner more vision, passing range and perhaps shooting ability/goal threat. I think we would be more solid and get more control and certainly more creative than any combination I can think of on the paper.
But we'll have to see at preseason if it can work or if Ole takes a gamble and not sign a CM/CDM we desperately need to give Garner a chance. Still a huge step up but looking at Matcior McTominay don't think he would do much worse.