While Garchomp's base Speed stat does drop from 102 to 92 when it Mega Evolves, it does **not** lose a flat 10 points in those stats. Instead, it loses a *percentage* of its Speed. And while I'm not going to go and write out 100 levels of percentages, I did test a few different levels. The Garchomp I used had 252 Speed EVs, 31 IVs and a neutral nature:

Speed of Lv 10 Garchomp: **34**

Speed of Lv 10 Mega Garchomp: **32**

*32 ÷ 34 = approx. 0.9411 = approx. 94%*

*approx. 6% drop in speed*

Speed of Lv 50 Garchomp: **154**

Speed of Lv 50 Mega Garchomp: **144**

*144 ÷ 154 = approx. 0.9350 = approx. 94%*

*approx. 6% drop in speed*

Speed of Lv 70 Garchomp: **213**

Speed of Lv 70 Mega Garchomp: **199**

*199 ÷ 213 = approx. 0.9342 = approx. 93%*

*approx. 7% drop in speed*

Speed of Lv 100 Garchomp: **303**

Speed of Lv 100 Mega Garchomp: **283**

*283 ÷ 303 = approx. 0.9339 = approx. 93%*

*approx. 7% drop in speed*

As you can see, when a Garchomp with the build that I specified in the beginning Mega Evolves, it lose 6-7% of it's total Speed*. If you want to know the exact numbers, do what I did and enter a vanilla Garchomp and a Mega Garchomp with identical stats into Smogon's Damage Calculator — it will show you. If you want to calculate the percentage just divide Mega Garchomp's speed by Garchomp's speed, and multiply your answer by 100.

_{* Without EVs the percentage is slightly higher, but not drastically. I encourage you t play around with the math yourself.}

I hope this helps!

P.S. Please please tell me if I messed something up, it's midnight when I'm writing this and I'm prone to errors.