In Let’s Go, Aerodactyl’s base speed is 130, without counting EVs. When it Mega Evolves, it’s base speed becomes 150.
However, if you EV train it, according to PokémonDB, it can reach a maximum of 438 at level 100! But, that happens with 252 EVs and 31 IVs both in speed.
If you want to calculate its stats with optimal accuracy, here’s a link
I don’t if the algorithm is perfectly similar to the one in Let’s Go, but it’s the only valid one I’ve found.