Data Kills Performance on Everything
We've all looked at graphs like this in hard drive reviews before. The drive's internal programming places data on the outer portion of the platter first since data transfers are faster on that area of the disk. As data populates the drive, new data goes further to the center of the platter, where the surface area doesn't rotate past the read head as fast. That means your HDD gets slower, and slower, and slower as you put data on it. The performance curve is an accurate representation of performance across the drive.
Then we have solid state drives. SSDs don't have platters, so there are no fast or slow areas of the drive. In graph above, we see high performance across all LBA space.
Sadly, in data storage, there are no fairy tales. The graph above is from the same SSD after a couple months of use as an operating system drive. As you can see, the performance drop is significant, even though the drive was cleared off all data to run the test. TRIM was disabled so the drive's performance during the test was similar to what an end-user would have during operation.
Data on Disk Testing with PCMark Vantage
Here we see the PCMark Vantage HDD Test Suite ran on the same drives above. One set of results were taken with the drives in a fresh out of box state and the other is with 50% of the drives populated with data.
Data does not discriminate; it kills performance on everything it touches. The more data you store on a drive, the slower the drive becomes, and it doesn't matter if solid state or mechanical.