The concept of being too slow relies on multiple variables that change on the format and rules you're playing with.
I would say that a Pokemon is too slow if even at maximum speed investment (Jolly/Timid with 252 EVs in Speed) it is outsped and OHKOed or 2HKOed by the most common sweepers and threats of that meta format. For example in the current Gen 8 Singles OU, Single Strike Urshifu's Wicked Blow is one of the most common moves. If a non scarfed, non banded standard set Urshifu Wicked Blow OHKOes or 2HKOes your own Pokemon, you're too slow and too frail, specially if you can't OHKO it on the one turn you survive (assuming 2HKO). Or the damage dealt by a Dragapult with Draco Meteor or Dragon Dart.
Of course that's not the only benchmark, but I'm using it as an example for you to take stock of the format you're playing in and familiarise yourself with the most dangerous and common sets and EV spreads. Clearly there is no need for you to calculate the speed and damage of every one of its threats, but if your Pokemon fails against the most standard un-choiced sets, then it's time to re-evaluate whether you really have to use that Pokemon. Besides, there's no way you can predict every single item and move your opponent has, but you're doing something wrong if you neglect to take into account some way of dealing with common threats.
Another consideration is whether your Pokemon gets access to priority moves or not. Having access to none puts it in a really vulnerable position at low health.
For example, an Azumarill is generally very slow. However, Belly Drum or Banded Aqua Jet sets can 6-0 or late game sweep (sufficiently weakened) opponents with ease, making it a fearsome force to be reckoned with.
Then comes the idea of whether the meta is conducive to Trick Room teams. Some meta is dominated Hyper Offense which punishes slower, more defensive teams by using brute power or Taunt to cripple your Trick Room setters, effectively shutting you out of the one win condition you have. So again, knowing the common offensive cores and being able to counter play against them will tell you what's worth using and what's not. Because in singles, your slow Pokemon has usually only 3 turns left to exploit the speed reversal. If you have to waste another turn setting up just to ensure KOs, you're usually better off using another strategy, as you might run into trouble more often.
Continuing on with the idea of setting up: If you're too slow and too frail (i.e. lack the defenses/bulk) needed to set up your win condtion (be it Swords Dance or Rock Polish or Dragon Dance or Curse or whatever) before your opponent can switch and OHKO or 2HKO you, then that set isn't viable in that format. Or if you're over reliant on a Weakness Policy to become an offensive threat, then you're in big trouble if the opponents move does 50% or more damage to you. Moves like Wicked Blow (again, just an example because I saw an OU match being swept late game by Urshifu sometime ago today) aren't a threat in tiers below OU and aren't strong enough to be huge threats in Ubers or AG. So knowing what moves are the bane to your Weakness Policy or Agility/Rock Polish sets and whether they're common in that meta, will also provide as an indicator to whether your Pokemon is too slow or not.
In conclusion, no amount of Shell Smashing will make Shuckle a reliably potent sweeper. Try to identify the biggest threats of the meta and compare your Pokemon with those, as there are literally 100s of niche picks that you can't predict, but at the very least you shouldn't fail to beat a kid who copy pasted 6 top tier Smogon sets and clicked buttons. So identify the strengths, weaknesses, and potential roles your Pokemon can play, and then check how the speed stat fits in all that.
And finally, accept that by virtue of design, some Pokemon are in fact, trash, and move on to others. Don't forget to have fun while playing, this is only a game (: