What's your fastest build?

Probably my Karla. Neutral spd, but I gave her L&D2 (because I don't want to use up a swift sparrow or fury on her) and she comes with spd wave so she reaches 50.

Though my Felicia can get pretty fast too. Neutral spd, 6 merges, fury, so 43 spd showing on the map, 2 from ally support with Nowi who might be using drive spd (other times she's using hone atk) and might be using rally spd/res, plus Marth is often around so that's another 2, but if he's not it's because I'm in arena and she's using spd ploy, so she'll often achieve 50 spd or 50 virtual spd.

Holy crap lmao, my Lewyn isn't even close. He is base 43 with Swift Sparrow, so 47 is my highest speed user.
A point of advice, I feel like 48/54 speed is overkill. You really don't need that much speed. I'd go for Fury tbh, 46/52 is (slightly) more reasonable. My Ishtar gets 46 speed, and she nearly always doubles, so it should be fine at 46/52.

Most of my fastest units just have their base sets so unbuffed I guess Flying Nino wins at 48 with Swift Sparrow, but Laegjarn could easily win attacking a buffed enemy.

Ishtar, I pulled a +Spd version a couple of months back. 39 base,but when she attacks she hits 49 speed, and with 2 Drive Speeds and Marth's refine she hits 57 then, her Support brings her to 59. No merges, but if they come, she will just keep getting faster.

How can I not show off my Karla? 50 spd, 56 on even turns, pretty sure I am going to get that +7 atk from her sword.

Well this is my Karla (+Spd -Hp) is not his original build but is nice she's reach 58 on Even Turns and well thats my Speediest Unit (Im guessing that with 3 Fjorms and Drive Spd it will be another story xD)