because of course it would be slower to gain pp the higher your total pp count is. Because osu has a weighting system.
For example: If you get 33 pp out of a map, you wouldn't immediately get that 33pp, osu would first check if it is in your top 100 plays, if it is, it will give the appropriate weight depending on it's placement, your highest pp play would be weighted 100% but the second would be 95% and so on until it reaches 1%. e.g: raw: 105pp top play and 100pp second top play after weight is added it would be 105pp top play and 95pp second top play.
So yeah naturally it would be easier to go from 0pp to 900pp than 900pp to something like 1500