If speeds were increased, then the tyranids' speed must also be increased proportionally or else you'll have a bad case of nids never being able to reach clawing distance. Sprint speed could use a boost but the same problem arises as marines would then be able to outrun running tyranids. Then if you boost tyranid sprinting speed to equalize it, marines wouldn't even be able to get a shot off before being clawed to death due to the running backward penalty. But if you remove the running backward penalty, then marines would be able to shoot too much and blah blah blah... it'd be a giant balancing mess.
Nailed it in one.