The spell in 5th edition
To dodge the attack, the Beholder must move, but stay within the confines of a space. One space = 5 x 5 square. Assuming he's standing in the center, then he moved roughly a foot to stay within said space.
Let's say its a low cloud at a height of 1000 meters, which seems pretty fair.
One Foot: .31 meters
So. Comparative speed.
0.31 meters moved x 443,842 m/s speed of lightning / 1000 meters lightning travelled
This equates to 137.59 m/s, or Subsonic. This is a suitable low-end, I believe.
Let's make another assumption. Let's say the cloud stands just above the group in general, at a height of 50 meters or so. Less likely, but it'll do for an example.
Changing our distance travelled by lightning to 50 meters, we get a new speed of 2751.82 m/s, or Mach 8.02, Hypersonic.
100 feet? So my high-end was a tad off. Our distance is 30.48 meters.
Plugging in that value we get a speed of 4514.14 m/s, or Mach 13.16. Just into Hypersonic+.
There's a notable class ability called "Evasion" that allows Rogues , Monks , and Rangersto completely avoid an attack if they make a save. Since the spell only targets things in a 5 foot range I'll use that as the distance they would need to cross.
100 feet / 1,443,569.55 fps = 0.00006927272 seconds
5 / 0.00006927272 = 72,178.485 fps or mach 64.1399 which is High Hypersonic+
1.524 x 440000 / 30.48 = 22000 m/s, Mach 64.14 as said above.
An important thing to note is that while other class such as the Fighter, Barbarian, Cleric, and Wizard can tag the above classes with the Evasion ability, they themselves cannot replicate their agility/speed. So while they may scale in terms of attack speed they don't scale in movement/general speed. Also Call Lightning is a 3rd level Druid spell, which mean that anyone 5th level or above can/would scale to all results but the evasion one and everyone 7th level or above would scale attack speed wise under the current system to my understanding.
Low End = 37.59 m/s or Subsonic
High End = 2,751.82 m/s, or Mach 8.02, Hypersonic
Result with spell description = 4,514.14 m/s or Mach 13.16. Hypersonic+.
Result with the Evasion feat = Mach 64.1399 or High Hypersonic+