Base 65 neutral natured with max investment secures the outspeed on a lot of key uninvested threats. For example, Glaceon gets 229 with 252 Modest and gets the jump on 44 Speed Rotom-W (which is in turn invested to get the jump on 252+ Azu). It however misses the jump on Adamant Brelooms and Bisharps just barely, who are really key. Timid nature allows you to get the jump on those two and 252+ (max invested Jolly) Tyranitar, and all uninvested base 100's like pixies (100 base across all stats) which is important as well. But you can go ahead and spec 252 into HP to increase bulk by a considerable amount and go Modest, it won't be too bad.