The OP is saying that the HD slowed down from what he remembers it being ... if he has a slow 5400 rpm drive then it would still have been a 5400 rpm drive back 7 years ago.
A 7200 rpm drive takes 21.2 seconds to boot up, the difference from 126 to 150 MB/s or 5400 versus 7200 rm can not account for 7 - 10 minute boot times. Like an old phonograph, the rate that the music comes through is dependent on only two things.... the density that the track is laid down and the rotational speed. Like the phonograph, while the mechanical design of the arm can improve reliability, put the needle on the LP faster and address other issues like handling vibration, only 2 factors affect how much sustained data gets read off the disk.
Firmware and cache can contribute to the initial burst and firmware for example can tweak access times in favor of other performance factors, but these primarily affect initial data reads. How long it takes to find the file, and the 1st few MB of data off it can all be improved by these factors, but sustained data reads are limited by dessity x rotational speed.
Data read = Data per unit length of track on the platter times the speed at which the platter moves under the head. The disk is typically twice as fast at the outer edge as the inner. We had over a 5 fold increase in attainable areal density in 7 years; not all of that made it into consumer drives.
So while areal density is the the biggest factor in sustained read speeds, it still does not account for 7 - 10 minute boot times.
And no, HDs to not actually slow down as data is added. Two things however affect how a drive performs.
1. The length of a track around the platter is typically more than twice as long as the inner tracks; therefore twice as much data is read per revolution at the outside of the platters as the inside. As more data gets added the more the disk is filled, and the more data starts getting laid on those inside tracks.
2. The more data that gets stored and removed, due to the HDs prioritization of where it puts files, it will fill empty spaces at the inside tracks first. This can lead to file fragmentation and to read a file the head may have to move to 3, 4 and maybe 5 locations on the drive to read the entire file.
The 2nd is easy enough to fix by defragging the drive. However this may lead to placement of large important files at the inner tracks. Those concerned about this slowdown, will utilize a partitioning strategy that prioritizes the placement of files on the drive. With no SSD in play to simplify things, this usually meant:
OS Partition
Page and Temp Files Partition
Games partition
Programs partition
Data partition
Of course the order and size of each partition, well typically all but the 1st two anyway, are user choice dependent and those using certain apps (video editing for example, typically has a "scratch" partition unless they had another drive for that purpose.