The slower the powder the further the bullet moves by the time the peak is reached, but the higher the pressure is further down the barrel, as when passing over a gas port. The powder is too slow only if that gas port pressure exceeds its spec limits and runs the mechanism too hard and fatigues or breaks it. This has been most notably a problem with the M1 Garand, whose long operating rod is particularly vulnerable to being sprung by excess pressure, but other guns can be run too hard, too, from a wear standpoint.
Note that because a heavier bullet does not move as far forward by the time a slow powder peaks, you have to use a lower charge weight than with a lighter bullet. That makes less gas, lowering gas port pressure again. So you can have a powder that is too slow for lighter bullets but that at the same peak pressure has the right gas port pressure for heavier bullets. For example, IMR 4350 is generally considered too slow the Garand system, and with the usual 150-175 grain bullet weight range, it is. But John Clarke had loads for 200 grain bullets with it that were OK in the Garand.