I disagree, then in your theory even your code is not fast enough to daytrade because there are much faster computer and internet speed available for high rollers.
Yes, forward testing is a good way to go but it takes more time. In fact, observing real time price discovery can be the best education you will get. Had to come back and add comments as the market was moving fast this morning.
If you don't try to compete on the sub millisecond level, their superior speed doesn't really matter as much.
I've been day trading manually for going on 19 years. I don't code but will if it becomes necessary. I cut my teeth on the old ER which had price action as fast and violent as any equity futures market today. To the OP, don't let someone's opinion determine what you can or cannot do...and that includes mine. Determine for yourself.
You need to understand the pecking order. You don't have to be the first one in, but you'll definitely be the last one in without a good program.
I don't think it's impossible. The problem is, your signal strength decreases, and risk increases as every second passes. HF firms have this perfected. A retail trader can be semi-perfect and still survive.