Recently, I played a few old RTS games (Tiberian Dawn, Tiberian Sun) and a few more recent ones (Red Alert 3, Grey Goo) and I was struck by how differently paced they are.
In the old games, everything happens slowly. You accumulate resources slowly. You build units slowly. Your units trundle across the map slowly. In Tiberian Dawn, for example, building even a single medium tank is a significant investment of time and money. Building a second tiberium refinery can effectively double your income, but it also means making yourself vulnerable for a long time if your opponent decides to put that initial investment into a rush instead. Everything happens slowly enough that you have time to act deliberately, and every action feels worth deliberating.
New RTS games, by contrast, feel like anxiety simulators to me. You rack up resources quickly. You churn through your build queues quickly. Units charge across the map. There's never enough time to do all the things I need to do. Oops, I tried to use proper combined arms tactics to assault an enemy base, but that stole my attention away from my build queue, causing me to ram my resource cap and now I'm pissing away credits. Oops, I tried to get my build queue in order and in the process my unit blob was left vulnerable and now the enemy's flanked me and destroyed my artillery. Oops, I tried to set up base defenses and while I was doing that my enemy beat me to that highly contested resource field by a few seconds.
When I lose in an old RTS, I feel like it's because I wasn't clever enough. When I lose in a new RTS, I feel like it's because I wasn't fast enough.
Well, that's kind of your answer why the old games feel slower to you.
It seems that the standard is setting these games to high speed. I don't think there is an argument to be made that newer RTS games are faster than that.
Who decides what "the standard" is and why should I care? Speeding up the older games is an option I don't have to use and one that is not set by default. I don't get the option to slow down the newer games.