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.