A few things need clarification on this topic. The most important thing I want to say is that if your site/application is running slow, you can ask DreamHost support engineers to try and move your account to a different server. This doesn’t cost anything and may solve your troubles. Check on the control panel which datacenter hosts your accounts.
That said, there are plans to keep on renewing shared hosting hardware, even if those plans are not public. Hardware in the datacenter is cycled regularly and new clusters are created with customer demand.
In this environment, fragmented hardware specs is very common. DreamHost has been growing and serving increasing amount of customers for 20 years: hardware expansions are planned at different points in time, every time by teams with more experience that leads often to making changes. The result is new datacenters, old datacenters, hardware of different generations, etc.
You need to keep into consideration the size of each datacenter and the amount of shared hosting customers on each one. If you only look at the list you miss important details.