Super-fast 5e monsters

If you know proficiency bonus, everything else is easy to find.

Attack bonus: proficiency x 2

Save DC: 8 + proficiency x 2

Damage: 5+(CRx5)

HP: 15+(CRx15)

"Good" saves/skills/checks: proficiency x 1.5

"Weak" saves/skills/checks: half proficiency, if any

AC: 10 + "Good" save.

Why? Well...

Attack bonus: double proficiency is meant to encompass both proficiency bonus an ability bonuses. These will be close to PC's bonuses.

Save DC: calculated like PC's save DCs.

HP and Damage: based on the DMG. Number of attacks is not that important. A limited attack ("recharge 4-6", etc.) affects a whole area (example: breath weapon) and, unless you succeed on a save, deal double damage (10+CRx10).

Good/weak saves: these are simplifications; monsters could have six different saves, but it is a decent approximation.

AC: start with unarmored. Since you don't add proficiency to AC most of the time, it is a bit lower than attack bonus. At high levels, monster will be easier to hit, but will be able to take more punishment as PC damage will not usually keep up with monster HP (at least for fighters).

Smoother table:

CR	Prof	x0.5	x1.5	x2
1	2	1	2	3
2	2	1	3	4
3	2	1	3	4
4	2	1	3	5
5	3	1	4	5
6	3	1	4	6
7	3	2	5	6
8	3	2	5	7
9	4	2	5	7
10	4	2	6	8
11	4	2	6	8
12	4	2	6	9
13	5	2	7	9
14	5	2	7	10
15	5	3	8	10
16	5	3	8	11
17	6	3	8	11
18	6	3	9	12
19	6	3	9	12
20	6	3	9	13
21	7	3	10	13
22	7	3	10	14
23	7	4	11	14
24	7	4	11	15
25	8	4	11	15
26	8	4	12	16
27	8	4	12	16
28	8	4	12	17
29	9	4	13	17
30	9	4	13	18

Special thanks: 

- Unknown user that commented here.

- Blog of holding.

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