Maybe it is just the typical "fast speed feels slower over time, because you adjust to the higher speed".
But, are there particular things that you do, that now feel slower?
You can also watch your Activity Monitor, to see if particular tasks cause a spike in CPU use, or result in higher memory usage. You might see the memory pressure graph go from green to yellow, or even to red...
And, also in Activity Monitor - you can see which processes are the highest CPU or memory users, either of those can legitimately cause system slowdowns.